You may have heard of frozen grapes as a tasty snack, but what about heating grapes in the microwave? You probably shouldn't try this one. It turns out that grapes react very oddly when they're microwaved.
Since grapes are full of moisture, when they are microwaved, that moisture turns to steam. If the stem is still attached to the grape and there is nowhere for the steam to escape, it will explode slightly and rupture the skin.
If you place two grapes close together with their stem holes facing each other, some arcing and sparking will occur. If there is just a single grape with its stem hole open, it will let out a stream of steam. Good thing we don't serve grapes warm.