Why does Uranus lie 'on its side'?

The impression that Uranus is lying 'on its side' arises from the fact that its axis of rotation is not vertical to its orbit around the sun, as in the other planets of the solar system. It is so strongly tipped that it lies almost parallel to its orbit. The probable reason for this is a collision with another heavenly body. This also probably gave rise to the narrow rings. Uranus goes around the sun once in 84 years. Since its axis of rotation is lying on one side, the poles point alternately in the direction of the sun every 21 years. There is then summer for 21 years and on the opposite side is winter.