Apples float in water because they are 25 percent air.
Why it is true
The internal structure of an apple contains microscopic air spaces between the fruit's cells. This composition makes the overall density of the apple lower than that of water, allowing it to float. This characteristic is the basis of the traditional game of bobbing for apples.
Source: USDA
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.