So, he opens the box and picks up the letter. He glances at it briefly to make sure it is stamped, and then puts it in his bag with his other letters. When he gets back to his office, the letter goes in a big bin with all of the other out-going mail. He thinks nothing of it, and finishes his day.
A few days later, he delivers the exact same letter to the same house that he picked it up from. He glances at the mailing address and observes that it is indeed the same as the return address. Anyone can make a mistake, so the mailman puts the letter in the mailbox so that the customer can readdress it for its proper recipient. He thinks nothing of it and finishes his day.
The next day, the mailman sees the same mailbox with the flag up. He opens the box and again sees the same letter, nothing changed, but with a new stamp on it. The mailman is perplexed, and thinking to save the customer both time and money, decides to ring the doorbell and inquire about the letter.
Ding-dоng, ding-dоng. Ding-dоng, ding-dоng. Ding-dоng, Ding-dоng.
The door opens and standing there is a stout Buddhist monk, dressed in traditional garb.
“Pardon me, sir,” the mailman says, “but you seem to be trying to mail this letter again, and without any changes to the address, it’s only going to end up back at your home in a few days.”
“Ah, my letter. Thank you so much for mailing it the other day, it was greatly appreciated. Please do so again” replies the Buddhist monk.
“But sir,” says mailman, “you will only waste a stamp, and this letter will be re-delivered to your home a few days from now.”
“But that is my intention, dear man,” replies the Buddhist monk. “You see, reposting is the best way to get karma.”

The first mathematician orders a beer
The second orders half a beer
« I don't serve half-beers » the bartender replies
« Excuse me? » Asks mathematician #2
« What kind of bar serves half-beers? » The bartender remarks. « That's ridiculous. »
« Oh c'mon » says mathematician #1 « do you know how hard it is to collect an infinite number of us? Just play along »
« There are very strict laws on how I can serve drinks. I couldn't serve you half a вееr even if I wanted to. »
« But that's not a problem » mathematician #3 chimes in « at the end of the joke you serve us a whole number of beers. You see, when you take the sum of a continuously halving function- »
« I know how limits work » interjects the bartender
« Oh, alright then. I didn't want to assume a bartender would be familiar with such advanced mathematics »
« Are you kidding me? » The bartender replies, « you learn limits in like, 9th grade! What kind of mathematician thinks limits are advanced mathematics? »
« HE'S ON TO US » mathematician #1 screeches
Simultaneously, every mathematician opens their mouth and out pours a cloud of multicolored mosquitoes. Each mathematician is bellowing insects of a different shade.
The mosquitoes form into a singular, polychromatic swarm. « FOOLS » it booms in unison, « I WILL INFECT EVERY BEING ON THIS PATHETIC PLANET WITH MALARIA »
The bartender stands fearless against the technicolor hoard. « But wait » he inturrupts, thinking fast, « if you do that, politicians will use the catastrophe as an excuse to implement free healthcare. Think of how much that will hurt the taxpayers! »
The mosquitoes fall silent for a brief moment. « My God, you're right. We didn't think about the economy! Very well, we will not attack this dimension. FOR THE TAXPAYERS! » and with that, they vanish.
A nearby barfly stumbles over to the bartender. « How did you know that that would work? »
« It's simple really » the bartender says. « I saw that the vectors formed a gradient, and therefore must be conservative. »