Anti-climbing paint
The wall surrounding Oxford Castle has anti-climb paint on it. At least that is what this ominous sign warned me of as I walked past it late one Friday night while I was stranded under the ash cloud. Of course, the rock climber in me was tempted to climb the forbidden wall; a few seconds later, the cops were yelling at me to get down, and I had to apologize so they wouldn't arrest me. "What is anti-climb paint, anyway?" I asked them as I ducked away into the darkness. "Does it work?" After all, I did get up the wall without much of a problem. The cops (who weren't really that mad) told me that anti-climb paint is kind of like pigeon glue; it has a long-lasting viscosity that makes surfaces sticky, slippery, and just plain unpleasant to perch on. It also sticks to human skin, so back in the day you'd be able to identify burglary attempts by checking the suspects' fingers. But, they said, "it might not be working anymore"; it's been a long time since people actually tried to break into Oxford Castle, which is now a tourist destination.
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