To add to steven's answer, "warming up" can also be metaphorical - like warming up (getting practice) for a test. In terms of temperature, I feel like "warming up" can also sometimes apply. For example, "can you warm up last night's dinner in the microwave?" is correct, but "can you warm last night's dinner in the microwave?" sounds weird. I sort of feel like warm up (also applies to heat up) in this sense, means bring the item up to a certain temperature, while warm / heat is just the act of making something be warmer / hotter. Maybe similarly "warming up for a test / exercise" is getting enough practice up to the level of the real deal. You can also "warm up" to someone or an idea, which means get used to and relaxed with a person / idea (so up to their temperature perhaps?). This is all just conjecture though!!

"Warming" is the process of adding heat to something. For example, "I was warming my hands over the fire." "Warming up" is a term to describe light stretching and exercise (e.g. jogging) to stretch out your muscles before a work-out.

Alice is correct that there are many different meanings when one uses "warm up" as a phrasal verb. However, when used to talk about heat, it is almost the same as warm. As Alice said, we generally use "warm up" to mean we want something to reach a particular temperature. Example: "Let me warm up the soup, and we can eat in ten minutes."

thank you everybody,.