The inside of the sleeping mat is open-cell foam; in essence, it is just like a bath sponge. If you were to squeeze a bath sponge flat in your hand, it would draw itself full of air once you released your grip. Exactly the same thing happens in a self-inflating mat, except that the 'sponge' has an airtight skin around it with a valve. Once the valve is opened, the foam core will draw air into the mat.
The sleeping mats are vacuum-packed at the factory to save on space and thus on shipping costs. This sometimes causes the individual cells of the foam to stick together, so that when the valve is opened for the first time, the mat may not inflate.
If you were to manually-inflate your mat (either by mouth or using a pump-sack), the cells inside would unstick and the mats will self-inflate the next time you use them.
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