My favorite analogy to explain the difference goes like this:

Imagine the pot is a hot tub and the bubbles are scantily clad people.

A simmer is when the water is warm and the people (bubbles) are just hanging out around the side of the hot tub.

A boil is when the water is so hot the people are jumping around the tub.

A rolling boil is when people are trying to leap out of the tub for fear of being burned alive!

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