Sparking fruits

When you cut a grape or other fruit in half lengthwise, but leave a skin flap connecting the two halves, the result is more or less the right length for it to act as an antenna for the microwaves. A modern microwave has a lot of power (about 1000 watts). The fruit anetenna is not connected to anything, so the energy its absorbing tends to bounce back and forth from one half to the other. The tiny skin flap connecting the two halves is a bottleneck, and within seconds, it heats up to the point where it bursts into flame. The flame contains ions from the fruit, and the flame can also absorb microwave energy.

Microwaves used for cooking are electromagnetic waves possessing frequencies around the 2.45 GHz range. Water molecules are absorbing the microwaves and start readily rotate gaining energy (temperature rising).