When I was about fifteen, I participated in a thirty-mile walk to raise money for charity. The final checkpoint was a pub, and of course everyone went into the beer garden and lay down on the grass.
Now you know how when you've been exerting yourself, you can walk fine until you stop, whereupon your muscles seize up. Well, after lying on the ground for a few minutes I got up because I needed to go into the pub and find the toilet, and of course I could hardly walk. So I hobbled towards the pub door.
A middle-aged man walked up and held my elbow, saying, "Let me help you, my dear."
First thought: wtf?! Why has this creep grabbed my arm without asking?
Second thought: Oh! In these baggy walking clothes, he thinks I'm a girl.
Third thought: Wait a moment. That means that girls get this sort of treatment all the time and I'VE NEVER NOTICED.
It was seriously a life-altering moment.