If you don’t know about grapes in the microwave, you haven’t been keeping up with the internet. “All right, so we’re going to do this grape in a microwave experiment.” For 20 years, people have been microwaving two grape halves connected by a bridge of skin to create sparks, light, plasma! “Well, there’s a lot of juice from these, actually.” A plasma is a superheated gas in which electrons have been stripped from the molecules. Lightning is a common example. So how do you get that from grapes? Amateur explanations focused on conducting electricity between the two halves. The skin bridge was the key. Or was it? Scientists who have now investigated the grape plasma phenomenon in depth say, no, you can use two whole grapes as long as they touch each other. And the explanation has to do not with conductivity, but with microwaves being trapped inside each grape. Here’s what happens. The wavelengths of microwave radiation are about five inches. But water, which is what grapes are made of, compresses these wavelengths to about a half an inch. That’s about the same size as the grape. So the microwave radiation bounces around inside the grape and concentrates in the center. That’s called resonance. With two grapes, or two spheres of a hydrogel — mostly water, but no skin — the concentration is where the two meet. There’s such a strong electromagnetic field at that spot that it sparks a plasma by stripping an electron off a sodium or potassium molecule. That triggers a cascade of other atoms in the air, losing electrons as they absorb microwave radiation. But that’s not what really excited the scientists. They were interested in the hot spots where the grapes meet. You can see them in red. Microwaves in that region were about 80 times smaller than in air. That was a real surprise to the scientists because water worked on microwaves in a way that had never been seen. Now if a substance could be found that would do that with visible light, which is so small it’s measured in nanometers, that could have great implications for microscopes, research in computer chips and other areas. So never assume a party trick is just a party trick, not if it involves a microwave. “O.K., that was awesome.”