Calvin and Hobbes by Bill Watterson for October 13, 1995
Calvin: As supreme earthling potentate, I'm afraid I can't let you just come in and take over the planet. Alien 1: You make this difficult. Alien 2: Prepare for war. Calvin: Let's not be hasty. I'll trade you Earth for 50 alien tree leaves. Alien 1: Pst! These primitive fools must use leaves as currency! Alien 2: It's a deal! Calvin: I need them by 8:00 tonight, and they have to be properly identified and labeled. Alien 1: Hurry, Nebular! At light speed, we can just make it! Alien 2: Our leader will be most pleased.