Crossing the street
When I was a kid, I learned that when passing a street I should walk perpendicular to the street, so as to minimize the time I spend on the dangerous street. It made sense to me that one should try to minimize the danger, but I did not feel that the perpendicular path was the optimal way to solve the problem if there were cars coming at a distance from only one side. I thought, and have been thinking all since, especially often when I pass a street, that one should walk a little bit in the direction away from the approaching cars. Now I have showed it mathematically, and I know which direction one should walk.