system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does microwaving continuously for sixty seconds heat food so much more effectively than two consecutive thirty-second cycles?

I noticed that my tea tends to be much hotter when I microwave it continuously than when I take it out half-way through. This result seems to be consistent regardless of material. Even if I open the microwave just for a quick second, whatever I’m microwaving needs significantly more time to heat. Why is this?
gpt
The magnetron inside your microwave that generates the microwaves is a vacuum tube, and the cathode inside it has to warm up before it starts emitting electrons. (literally warm up, as in temperature) 

You will usually be able to hear a difference in the noise it makes a few seconds after you start it, this is when it's really starting to work.