Memory foam is mae from polyurethane with additional chemicals that increase its viscosity level, thereby increasing its density. It is often referred to as visco-elastic polyurethane foam. Depending on the chemicals used and its overall density, it is firmer in cool temperatures and softer when warm. Higher density memory foam reacts to body heat which allows it to mould itself to the shape of a warm body within a few minutes. A lower density memory foam is pressure-sensitive and will mould more quickly to the shape of the body. The example often used to demonstrate its properties is that a hand pressed into the foam and then removed will leave a clear impression in the foam.