If you are caught in the rain, will you get wetter running to shelter, walking, or standing still?
This is the sort of thing I think about for the half-second it takes me to gear up for the dash to shelter when I'm caught in the open by one of Toronto's sudden downpours.
Standing still, I present a relatively small surface area to the falling rain -- in a vertical downpour, just my head and shoulders. When I move through the vertical rain, I collide with water droplets that would otherwise have passed me by -- but the same amount of water should strike my top surfaces (I move out of the paths of some droplets, but into the paths of others). Standing still wins in the first approximation.
On the other hand, I'll be in the downpour for a shorter period of time, so fewer droplets will strike my head after all. Running wins in the second approximation.
Jesse, of The Virtuosi derives mathematical support for the running side of things, concluding that "if you are Usain Bolt, you can reduce how wet you get by almost a factor of two by going from a meander to a sprint!"