Which planet would float, given a big enough bath?

Saturn's average density is about 0.69 grams per cubic centimetre, less than water, so the schoolroom image holds - though it is a thought experiment rather than a fact, since a bath that size would have its own gravity and the planet would not survive the dunking.

It is that light because it is almost all hydrogen and helium and enormously distended. Saturn is nine times the Earth's diameter with only about 95 times the mass, and it spins fast enough - a day of ten and a half hours - to be visibly squashed, about ten per cent wider across the equator than pole to pole.

Its oddest feature sits at the north pole: a cloud pattern in the shape of a nearly perfect hexagon, about 30,000 kilometres across, first seen by Voyager in 1981 and still turning. Nothing else in the Solar System looks like it. Laboratory experiments with rotating fluids can produce polygonal shapes at the right speeds, which is the leading explanation, and it remains one of the strangest things anyone has photographed.