If i throw a button at a car window will it break?

Will the button break? Or will the window break?

It depends at the speed of the car and the material of the button. A small metal object hitting a car windscreen at high speed can break the window.

A plastic button hitting the window of a stationary car will probably not do much harm, but a plactic button hitting the car window of a moving vehicle may do a lot of harm, not necessarily by breaking the window but by distracting the driver, which in turn may cause a fatal accident.

Suggestion: Don't do it!