system
Given the following post, respond as if you're commenting on Reddit:
human
When I pour sugar into microwaved water, why does it fizz, whereas when I pour sugar into water boiled on the stove it does not?
gpt
The likely reason would probably go like this:

Tap water tends to contain a decent amount of dissolved air in it, and gases are less soluble in hot liquids than cold ones. So boiling water removes most of the dissolved gas, which you can actually see if you look closely at boiling water, as there are no small bubbles around once it's boiled for a while. 

If you heat water in the microwave, it heats up much faster. It'd also typically be in a smaller container, with ~~less~~ more surface area for the bubbles to form (nucleate) on. For which reason you have to be a bit careful with microwaving water, as it might not form steam bubbles (i.e. boil) at all either. Instead becoming superheated, only to suddenly and dangerously flash-boil once you stick a spoon or something into it. So you're more likely to end up with a significant amount of air still dissolved in your microwaved water.

When you then pour the sugar into the hot microwaved water, you're suddenly providing a whole lot of nucleation sites and allowing the still-dissolved air to escape in a fizz, while in the stove-boiled water it's already escaped. 

It's much the same thing as the famous menthos-in-coke phenomenon, except that it's a small amount of air and not a large amount of CO2 being released.