What process cause such stone to became smooth?

Question

It's on my homeow3k I need help

Answer 1

Stones and other rocks can become smooth through a process know as weathering. Especially rocks found in river beds because they have a constant rush of water over their surfaces resulting in smooth stones. Keep in mind this is a very long process and does not happen overnight. 

 

I hope this helps helps! If not, try googling 'weathering' or 'erosion.'