Tension Mark

So, in university, my girlfriend asked me, "What's a tension mark? How do I make a tension mark?"

"What?"

"A tension mark. I need to put in a tension mark."

"A... tension mark? There's no such thing as a tension mark. Do you mean, like, an exclamation mark?"

"I'm supposed to send an e-mail about a job, and they said I need to put a tension mark in the header."

I couldn't stop laughing once I realized what a tension mark was.