Space
Saturn is so light for its size that it would float in a big enough bathtub.
Saturn's average density is about 0.69 grams per cubic centimetre, less than water. It is the only planet in the solar system less dense than water.
Why is this true?
Saturn is made mostly of hydrogen and helium, the two lightest elements, wrapped around a relatively small rocky core. It is huge, but it is mostly gas.
More facts in this category
More about space
Space facts are mostly a lesson in scale. Days longer than years, storms wider than Earth, light that left its star before the pyramids existed — the numbers stop being intuitive very quickly. Every entry here comes from established astronomy and includes a short explanation of what causes the effect, so the fact holds up when someone asks the obvious follow-up question at the dinner table.