When I was a teenager, I'd lock myself in my bedroom for hours every night. One night, my dad kicked the door down to find out what I was doing. He walked in to discover me sitting on my hand. "Ah, that old trick," he laughed. I gave an awkward smile back. "I have to say, son," he continued, "I'm relieved to be honest. I thought you might've been doing something weird. I'll leave you to it." When he closed the door behind him, I just shrugged my shoulders and carried on fisting myself.