What happens if you take away all the air pressure from the outside of a marshmallow? It gets bigger. Normally, 14.7 pounds per square inch of air pressure is pressing on the outside of the marshmallow – and 14.7 pounds per square inch of pressure is pressing outward from the tiny air bubbles inside the mallow.
If you remove the pressure outside, all those tiny air pockets inside start to expand and make the marshmallow grow bigger. Now the sugary goodness that is the marshmallow can’t expand forever. At some point, the air bubbles inside begin to escape and the mallow will stop growing. In fact, it will being to shrink slightly as the pressure that made it grow is now reduced since the air inside has escaped. This is the ideal time to allow the air to rush back inside the vacuum chamber. As those air molecules speed back in, they slam into the soft outer surface of the marshmallow and actually compress it a little bit. The end result is a slightly smaller and wrinkled blob of sugary goodness. Still edible, but not quite as fluffy.
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