Empirically delicious! Well done.

I wonder how big of a role the material plays... anybody want to try this experiment with some of those fancy stainless dice and get back to us with the results? ;-)

@AC, probably nothing would happen (still): cooking.stackexchange.com/questions/79346/… Given their simple geometry, the only case where I think microwaving dice would have some effect is if they were made of food or other food-like objects, instead of their usual materials...

...though maybe a D4 would be pointy enough to get some arcing?

@Icyfire Candy-dice! I love this... Even if I don't think it will last more than one round

I joined rpg just to upvote this answer. Well done!

amazing answer, too bad I can't +2

Possibly worth covering: there's a common myth that microwaves heat from the inside out, which could give the impression that we can melt the core of the die without affecting the outside. But microwaves heat from the outside in like anything else, so the outside would start melting before the core reached melting temperatures.

This is like the Mythbusters of tabletop rpg.

I think there may be a subtle fallacy with this answer. A die which was already biased to turn up 18 was tested, and it continued to show a bias to 18. But, there's a probability for each face - after the microwaving, all we know is that the probability of rolling 18 is still non-negligibly higher than that of any other number. It cannot be said conclusively from this that the overall distribution has not been changed (what's the probability of 19?). If the same test is run with a die that shows no initial bias, a bias may or may not emerge, and this test doesn't seem to tell us either way.

@DarrenRinger This experiment has a lot of problems, true, but I understood "significant" as gameplay significance, not statistical significance. The bias toward 18 isn't noticeable in gameplay, so if microwaving can't overcome that bias, then it likely would not be noticeable. I actually think the fact that the die didn't heat much at all is stronger evidence that this doesn't work, as discussed in the second section.

It's not really true that microwaves heat from the outside in or the inside out, nor that they heat from a certain direction, which seems to be the assumption underlying the rumor cited by the OP. Inside a microwave oven, you get a three-dimensional standing wave pattern. If you google on "microwave oven standing wave pattern" you'll see a variety of idealized and real-world patterns. The die is simply immersed in this pattern. For simplicity, you could imagine a one-dimensional standing wave, which is a superposition of waves traveling in both directions. There is no direction of incidence.

When doing this, roll a DC 10 Household Electronics Safety check.

If you microwaved it in a dish of water to part depth, you could cause localised heating and possibly distortion. A dish of oil would be a bad idea but would allow you to get hotter, and would lead to many more materials being deformed.

@BenCrowell but within that standing wave pattern, a solid object that's capable of absorbing the waves will absorb more on the outside than the inside (because the energy absorbed by the outside is no longer available to heat the inside)... so the net effect is that microwaves do indeed heat things from the outside in. The only counterexample would be an object that has low absorption in its outer layers but high absorption in its core; the microwaves would pass relatively unhindered through the outer layers and heat the core -- from the outside of the core material in ;)

@ChrisH I was going to suggest this. Just a dish with 1/8" (3mm) of water or less. While the die wouldn't absorb much and heat itself, the surrounding water would, and thus potentially warm the lower part of the die enough to warp it.

Have your 100th upvote (and probably a hat). You took this question and made it well done.

@DoktorJ: Interesting point, but I don't think your analysis is quite right. For the reasons explained in the answer, the absorption of microwaves by the die is very weak. Therefore the intensity of the standing wave pattern is not changed significantly by absorption.

The problem is, that's one die formulation. Problem 2 since plastic in and of itself is usually not going to respond the die may have to be placed in something (say veg oil) that can melt the plastic.