
Heat up
Common Foods
Use 'heat up' for usual items like leftovers, not raw ingredients to cook.

I will heat up last night's pizza for lunch.
Quick Process
'Heat up' implies a fast way to make something hot, not long cooking times.

Can you heat up the muffins for a minute?
Microwave Usage
It's commonly used when referring to microwave heating, but not exclusively.

Just heat up the stew in the microwave.
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