Which planet spins on its side?

Uranus, tipped over at about 98 degrees - so it rolls around its orbit rather than spinning like a top. Almost certainly something very large hit it early on, and its moons and rings orbit in the same tipped plane, which suggests whatever happened happened before they formed.

The consequence is the most extreme seasons anywhere. Each pole spends 42 years in continuous sunlight and then 42 in continuous darkness, and at the solstice the Sun sits nearly overhead at the pole - a planet where, for decades, the pole is the hottest place and the equator the coldest.

It is also the coldest planet in the Solar System, colder than Neptune despite being closer to the Sun, with a recorded minimum around minus 224 degrees. Neptune radiates significant internal heat and Uranus barely does, and nobody is certain why - possibly the same collision disrupted the internal structure that would otherwise carry heat outward. One spacecraft has ever visited, Voyager 2, for a few hours in 1986.