@Belkin That's a tough question to answer. To be honest, many big-minded people, myself included, think about that tirelessly day and night.
Maybe it's pure magicity of monster girls? Or maybe any girl will do? Sometimes even rocks can make your pee pee hard, but that heavily depends on various conditions, such as:
- shape
- moisture level
- which compound of basalt this is
But let's move away from rocks to lizards.
Lizards love rocks, that's what they use to heat up their blood systems during their daily existence in a desert. And that's why lizards are hot.
But why should hotness cause hardening of pee pee? Good question, my fella!
Hot blood increases blood pressure, filling your pee pee with blood, which then acts out as a hydroskeleton and in turn provides pee pee with semi-rigid structure.
The complexity comes from detail. Up to this day there is no scientifically significant computer model of why things make your pee pee hard. Of course there are differential equations, that tell you what could cause your pee pee go hard, but they are still limited to very rough estimations. And what about computer models? They don't quite work; there's just too many parameters to consider. Pee pee hardening even involves blood coagulation, which is quite important, when you're dealing with blood hydrodynamics and whole mental response.
Right now the best prediction of pee pee going hard gives us artificial intelligence, which is specifically trained to feel stuff we humans do.
For that we attach electrodes all over test subjects' pee pees and run electricity through a feedback loop, that is directly closed through a high-performance recurrent neural network.
But as of now, we only could achieve couple comas and 96% prediction success rate.