Good. As you can see, the square is empty again.

That's the basics of entering numbers. There is a feature called "pencil marks" that can be very handy on some of the more difficult puzzles. The next page will teach you how you can use them.

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Oops - you didn't press your second number.

Press your second number again.

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Oops - you tried to select a different square. I selected the original square for you.

Press your second number again.

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Notice that the first number you pressed has been replaced with a second number.

And finally, if you press your second number again, it will be removed from the square, leaving the square empty again.

Press your second number again.

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Oops - you pressed the same number again. I need you to pick a different number, okay? :)

Press your second favorite number between 1 and 9.

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Oops - you tried to select a different square. I made the original square selected again for you.

Press your second favorite number between 1 and 9.

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And now a number shows up in that square. Let's pretend, however, that you actually meant to type a different number instead. Since the square isn't empty anymore, pressing a different number will change the first number you pressed in that square to the new number.

This time, press your second favorite number between 1 and 9.

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Now that an empty square is selected, pressing a number key will put that number into that square. Let's see this in action.

Press your favorite number -- between 1 and 9, of course!

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You'll need to select an empty square, since you can't enter numbers into squares with numbers given to you.

Select an empty square by clicking on it or using the arrow keys.

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Entering numbers into the grid is pretty straightforward, but there are some handy features you should know about.

First, like I mentioned in the last chapter, you need to select an empty square before you can enter any numbers. Go ahead and do that now.

(Remember, you can select a square by clicking on it, or by pressing the arrow keys.)

And now all of the numbers in that square have been removed. Don't worry - if you accidentally delete the numbers from a square, you can use undo to bring them back. Check out the page in the next chapter on using undo and redo for more info.

The next chapter will show you how to use some of the other features like undo and the lock.

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Oops - you tried to select a square that you can't change.

Select one of the squares that has one or more numbers in it, then press the Delete key to clear the square's numbers.

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Oops - you tried to enter a number. For now, I'd like you to just press the Delete key.

Select one of the squares that has one or more numbers in it, then press the Delete key to clear the square's numbers.

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Oops - that square is empty! Try picking one that has some numbers in it.

Select one of the squares that has one or more numbers in it, then press the Delete key to clear the square's numbers.

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You can see that the square now has only the number you pressed in it.

One last note - you can also quickly clear all numbers from a square, whether it has pencil marks or not. To do this, just press the Delete key.

Select one of the squares that has one or more numbers in it, then press the Delete key to clear the square's numbers.

(Remember, you'll need to pick a square that you can change - it'll have a green background when it's selected.)

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Oops - you weren't pressing the Ctrl key. When you press a number key, make sure you're also holding down the Ctrl key at the same time.

Press Ctrl + a number to set the square's number.

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Oops - you cleared all of the numbers from the square. I put them back for you.

Select one of the squares that has more than one number in it, then press Ctrl + a number to set the square's number.

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Oops - you didn't pick a square with more than one number.

Select one of the squares that has more than one number in it, then press Ctrl + a number to set the square's number.

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The square is back to a single number.

You'll often discover what number a square should have in it while it has numerous pencil marks. A way to quickly set the square to that number is using the Control key. Simply hold down the Control key while you press the number (it might be labeled as Ctrl). All of the pencil marks in the square will be replaced with the number you pressed.

Select one of the squares that has more than one number in it, then press Ctrl + a number to set the square's number.

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Oops - you cleared all of the numbers from the square. I put them back for you.

Press two of your three numbers again (your first number, your second number, or your third number).

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Oops - you selected a different square. I've reselected the other square for you.

Press two of your three numbers again (your first number, your second number, or your third number).

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Oops - you pressed a completely different number. For now, I'd just like you to press two of the three numbers you entered in the last few steps (your first number, your second number, or your third number).

Press two of your three numbers again.

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Now you can see that the square has your three numbers in it.

You can remove any of the numbers by pressing them again. Try this out by pressing two of your three numbers again.

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Oops - you set the square's number using Ctrl. I'll talk about that in a few steps. For now, just press the number key by itself.

Enter a third number, other than your first number and your second number.

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Oops - you cleared all of the numbers from the square. I put them back for you.

Enter a third number, other than your first number and your second number.

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Oops - you selected a different square. I've reselected the other square for you.

Enter a third number, other than your first number and your second number.

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Oops - you pressed one of the same numbers again. I need you to enter a different number.

Enter a third number, other than your first number and your second number.

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Notice that the first number that was already there shrank to make room for the second number.

Enter a third number, other than your first number and your second number.

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Oops - you set the square's number using Ctrl. I'll talk about that in a few steps. For now, just press the number key by itself.

Enter a number other than your first number.

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Oops - you cleared the first number from the square. I put it back for you.

Enter a number other than your first number.

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Oops - you selected a different square. But don't worry - I've reselected the other square for you.

Enter a number other than your first number.

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Oops - you pressed the same number again. I need you to enter a different number. :)

Enter a number other than your first number in the same square.

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Like you saw on the last page, a number was added to that square. Since pencil marks are turned on, entering another number will add that number to the square, instead of replacing the number.

Enter a number other than your first number in the same square.

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Oops - that square wasn't empty. I deselected it for you.

Select one of the empty squares and enter a number.

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On some of the harder puzzles, you'll want to keep track of what numbers each square can and can't be. To record these numbers, you can use "pencil marks."

In the puzzle to the right, you can see that some of the squares contain more than one number. Those extra numbers are called pencil marks. They indicate that based on what you know so far, it's possible that any of those numbers could be the correct number for that square. Let's go over how to enter your own pencil marks.

On the previous page, there was no way to put more than one number in a single square. On this page, however, I've turn on the pencil marks setting. You can read about how to turn on pencil marks in the last chapter.

Select one of the empty squares and enter a number.