The molecules in the icecube (You didn't specify what it was that was melting so I'll just call it an icecube) start to move further away from each other. When they are frozen, the molecules are tightly bound and do not move much, however when the ice is melting, the molecules move more and more until they become liquid water. If you leave the water out for long enough, the molecules will actually leave the water and you will find yourself without any water to drink! haha hope this helped!
