You have two apples and you give one apple to your friend. How many apples do you have now?

$${\mathtt{2}}{\mathtt{\,-\,}}{\mathtt{1}} = {\mathtt{1}}$$

You should know this, but...

Here are some things that might happen:

1. A thief (including your friend) might steal the apple.
2. Your friend might give it back to you.
3. Your friend might eat half of it and give it back to you.
4. Your friend might give the seeds back to you.
5. You might eat the apple you're left with.

So there's more than 1 answer.