Plank noticed that, after a practice or a game, the cotton T-shirt that he and the other players wore under their pads would be heavy, soakedwithsweat.
That's convenient for people who live in their headgear 40 hours a week, but it's also nice for companies where employees share helmets -- after all, it'd be nice to take out your own lining so that you're not sharing the same sweat-soaked padding with someone else.