Most mice if they have a rubberized finish it can come off over time. I am sure that over time mice incur many scratches but they are mostly cosmetic. And it is possible over time for the plastic shell of a mouse to become slightly discolored by sweat, dirt, etc... Though I have never heard of a mouse melting from normal use obviously if you were to put it in an oven it would melt but not from proper use. Accept for this one time that I got very angry at an online home work assignment an threw the mouse at my bed but it hit the steal bed frame and shattered the left click button.