When I came across this image, I thought it was a joke or someone's attempt to create "art."
Turns out, it had an illustrative purpose. Apparently, in Baldwin Park, California, it is illegal to ride a bicycle in a swimming pool.
Note that I used the present tense: "it is illegal." Yes, that law is on the books, though it's (thankfully) not enforced and no one is sure of whether it ever has been.
From what info I've gleaned, the law against riding on or in was passed in the 1970s, when BMX cycling and skateboarding were popular, mainly among adolescent and young adult males. The real purpose of the law, I think, was not to keep kids from pedaling in their families' backyard swimming pools. No self-respecting teenaged boy in California (or most other places) would have done such a thing. Rather, I suspect that the law was passed in response to complaints after those young rebels broke or cut into fences surrounding larger pools.
But the young and restless weren't looking to turn their bikes into amphibious vehicles or their skateboards into water-skis. Instead, they broke in during the fall and winter, when those pools were drained and became, in effect, rinks. So, as often as not, the owners of the properties didn't discover the "crime" until weeks, or even months, after it was committed.
I strongly suspect that at least some of California's current law-makers and -enforcers broke that law at some time in their youth. And that is the reason why the law hasn't been repealed: Part of the fun of being an adolescent is rebelling against something (or, at least, feeling as if you are) and getting away with it. So, while living in such a mindset, what could be better than breaking a law and knowing that you most likely will get away with it. And what loving parent wants to deny their kid that pleasure?