Saturn would float in water if there were a bathtub large enough.
Why it is true
Despite its gigantic size, Saturn is the only planet in the solar system with an average density lower than that of water, due to its mostly gaseous composition of hydrogen and helium.
Source: NASA
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.