Spoiler: the watermelon explodes when you run 20,000 volts through it

"What happens when you pump 20,000 joules into a watermelon?" rhetorically asks The Backyard Scientist. "Two words. Pink Mist."

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1. 20,000 joules is not 20,000 volts.

2. I don't think the typo hertz anything

3. Back in the 1960s experiments were being done to see if mining explosives could be replaced with, in effect, water filled tubes with electrodes through which a significant electric discharge was applied. So inevitably we had to try it at school.
20kJ is quite a lot. We never got beyond about 1000uF at 400V - about 80J - but that could punch holes in thin aluminium sheet and explode apples. So 20kJ in a watermelon would be expected to be pretty dramatic. In fact, I wouldn't like to try and contain it in anything that might shrapnel, including wood.
(The mining experiments worked but it was too expensive compared to the mix of fuel oil and ammonium nitrate that forms a lot of mining explosive.)

4. An opportunity for teaching here*. Electricity does not just take the path of least resistance. Current distributes itself according to the resistivity and the path length. As a melon has about the same resistivity throughout the soft part, you will get more current density through parts close to the electrodes but some current will flow throughout the melon. Depending on the electrode separation and the voltage, of course, an arc may be struck between the electrodes and then almost all the current will flow through the arc. I haven't recorded my experimental setup from the 1960s, but imagine a small apple with electrodes in each end and a couple of pins in between connected to an oscilloscope.
I offer this thought to anybody who might want to measure current distribution in electrocuted melons in the hope of getting an IgNobel.

*Unasked and I expect unwanted, but old habits are hard to break.

5. Ohm my god. Watt are you taking about?

Sorry, I couldn't resist.

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