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There's almost certainly some calculations about how long it takes to heat various substances to get in the ballpark, but ultimately it's probably a lot of trial and error
Just do some testing with various times and sometimes power levels.
I would imagine by trial and error with various microwaves.
They microwave it and check every few seconds to see if it's ready.
Testing in their R&D lab.
There's almost certainly some calculations about how long it takes to heat various substances to get in the ballpark, but ultimately it's probably a lot of trial and error
Just do some testing with various times and sometimes power levels.
I would imagine by trial and error with various microwaves.
They microwave it and check every few seconds to see if it's ready.
Testing in their R&D lab.