What happens to a crisps packet on an airplane?

Bags of chips (crisps) can expand in an airplane.

A bag of chips on the ground doesn't appear to be inflated because the outside air pressure is high. As the elevation increases (like in an airplane) the outside air pressure decreases, which means that there is also less pressure in the bag. As the pressure decreases in the bag, the gas inside wants to take up more room and it expands (similar to what happens when the pressure is released from a can of soda).

However, the cabins of most airplanes are pressurized, so you would not be able to see any expansion.