Okay, I saw this first on twitter, here. Warning: it's cool, but kinda icky. First, get a watermelon.
Cut in half lengthwise.
Shave off the green skin, leaving the white part.
Start cutting with a sharp knife.
Make brain-like folds as shown.
Mine looks kind of gruesome because the white rind was so thin that the red part showed through, making it look a bit too "bloody" for me.
But I guess it does look kind of brainlike. It creeped out my family! But they still ate it. And you know me, I had to make a mini:
Out of a radish. ^_^