How does a self-inflating camp mattress inflate itself?

Self inflating camping mattresses are filled with open cell foam. Open cell means the air in the bubbles is not permanently trapped but can be pushed in and out of the bubbles. This foam is made from a flexible and durable plastic with a strong "memory" so that it tries to return to the same shape after pressure is removed.

When the mattresses air vents are opened on a deflated mattress, the pressure of the foam trying to revert to its expanded shape pulls air in to the mattress. The vents are then closed and the trapped air inside the mattress provides support for people lying on the mattress.