“Boil and Pop” refers to the act of immersing a plastic figure in boiling water. After about 45 seconds, the softer PVC plastics (usually used in heads and limbs) will soften enough that they can then be “popped” out with a good strong tug. This is a good way to remove parts (usually heads) while preserving a joint mechanism. It’s not 100% guaranteed, but it often gets results. Just to be safe, I use an old pot that won’t be used to cook food.