Contrary to what you might think, Popeye the Sailorman had it all wrong. In the old cartoon, whenever Popeye is getting beaten up he eats some spinach and becomes really strong. His muscles bulge out and everything.
In all actuality, eating spinach has nothing to do with strength. See, some time in the 1920s it was reported that a half cup of cooked spinach has thirty-four milligrams of iron, which is a lot. Iron is important because it carries oxygen in the blood. We need oxygen for energy; without enough of it people become weak and tired.
It turns out that the thirty-four milligrams was a typo. A half cup of cooked spinach really has only three point four milligrams of iron. Plus, spinach is actually worse than other iron-containing vegetables because it has a chemical that blocks most of its iron from being absorbed by the blood.
But when the typo was published the idea caught on that spinach was a muscle-building vegetable. That's where the idea for Popeye came from.