Hubcaps/wheeltrims why do they look like they are going backwards?

[Athley]

When you see a car why does it look as though the wheel trim/hubcaps are going backwards when the car is travelling forwards.

We are 3 women trying to figure this out please help?

[Mattk]

an optical illusion.when something rotates at a certain speed it appears to be going the other way.check out a fan or propellor for instance.