"What makes you so sure?" she'll ask.
You'll shrug and say, "Just trust me on this." Then tell her to chain you to the couch when the full moon hits.
On the night of the full moon, your girlfriend will chain you to the couch, then she'll sit and watch you.
"How do you feel?" she'll ask.
"Okay," you'll say. Then you'll add, "Wait, actually, I don't feel so good. I feel like something's about to happen to me." Your girlfriend will wait in breathless silence. Then you'll fart.
"Jerk," your girlfriend will say.
"Sorry."
The next day, you?ll both wake up exactly where you spent the night, her in the easy chair and you chained to the couch.
"I guess you're not a werewolf," your girlfriend will say.
"Guess not," you'll concede.
"Then you must be able to remember where you spent the night last month when I couldn?t reach you at home," she'll say.
The werewolf thing didn't work and you?re out of ideas. You're going to have to tell her about the affair.
Happy Tell Your Girlfriend You're Worried You Might Be A Werewolf Day!