Never set your laptop on a blanket, your bed, a cushion, or anything soft while using it. This can cause serious damage as it blocks the intake and exhaust fans located on the bottom of the laptop. The fans work too hard to move air through the system to keep it cool and can eventually fail, causing the laptop to overheat. In some cases you will notice slow-downs, loud noises, crashes, freezing, and even permanent damage where the only repair is no repair, just replacement of the laptop.
It should always be on a hard, flat surface to allow air to move underneath and through the grills over the fans to keep the processor and the rest of the inside cool and quiet.