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In my physics class today, we were using a bell jar and a vacuum pump to further demonstrate the combined gas law.
As the vacuum pump removes the air from the bell jar, the marshmallow expands. This is basically the reverse of blowing up a balloon... instead of increasing the pressure inside, we're decreasing the pressure outside. Both processes increase the pressure differential. When the marshmallow gets large enough, there's a small breach in the surface, which allows the pressure in the marshmallow to equalize with the pressure outside.
When the air re-enters the bell jar, the inside of the marshmallow had been at a lower pressure and the volume decreases as if that were its original pressure... leaving the marshmallow void of most of the air that was once inside of it.
TL:DR - Peep + Vacuum Pump = Deflated Peep