Today, at the start of the (hopefully) warm summer days, the MopDog presents to you yet another mental image that cannot be unseen... or rather, in this case, untasted.
Buying watermelons is somewhat of an art form. I was taught to knock on them, and listen for a deep, echoing sound, to make sure the melon is ripe but not too watery. They also have to have a yellow "belly," and they have to be really, really large, with nice stripes on them. Before the EU it was also allowed to ask the seller to poke a hole in it for a taste, but obviously it was not in the seller's best interest.
Watermelons are grown on fields, and stealing them had probably been a popular pastime through the ages. There is a joke I have heard from multiple sources about the nameless farmer who wanted to keep thieves away by putting up a sign that said "ONE OF THESE WATERMELONS IS POISONED." He returned the very next day, and found the sign crossed out and a correction scribbled on: "TWO OF THESE WATERMELONS ARE POISONED."
Okay okay, we are getting to St. Lawrence. This very popular saint in the Catholic canon has his feast day on August 10th. This, incidentally, is also around the time people have noticed watermelons start going watery (duh) and tasteless, and they are not as sweet and fresh as they are earlier in the summer. Hence the folk wisdom was born:
"St. Lawrence pees in the watermelons."