Hello. If you give a styrofoam cup to pressure cooker, it expand first, but than it extremly reduce. It reduce because the high pressure of the water vapor within the pressure cooker compresses or squeezes the tiny air pockets within the styrofoam cup and reduces the size of the cup, ok. But why it expand first? And why it will melt, if you leave it in the pressure cooker too long? Thank you very much.