Are these tennis balls yellow or green ... they are a type of "fluorescent yellow" that can also be seen as green. So why do some people see different colours? The answer may lie in a previous ...

If all tennis balls have a nap, why do players bother looking at them ... court—and for people to argue whether it’s yellow or green. Have you got a Big Question you'd like us to answer?

They are often competitive casualties cast aside after a match, but tennis balls are bouncing back to life thanks to a recycling center as big as a ball machine. Meet the Green Machine.