Two jokes, from sickjokes.about.com:
1)
A woman walks into the drugstore and tells the pharmacist she wants to buy some arsenic.
"What do you want with arsenic?" asks the pharmacist.
"I want to kill my husband because he's cheating on me with another woman," the lady replies.
"I can't sell you arsenic so you can kill your husband, lady, even if he is cheating on you with another woman," the pharmacist says.
The woman then reaches into her pocket and pulls out a picture of her husband with the pharmacist's wife and hands it to him.
"Oh, I didn't realize you had a prescription," the pharmacist replies.

2)
Three tortoises, Troy, Andy and Wayne, decide to go on a picnic.
Troy packs the picnic basket with beer and sandwiches. The trouble is that the picnic site is ten miles away so it takes them ten days to get there.
When they get there, Troy unpacks the food and beer.
"Ok. Wayne, give me the bottle opener"
"I didn't bring it" says Wayne
"I thought you packed it"
Troy gets worried, He turns to Andy, "Did you bring the bottle opener?"
Naturally Andy didn't bring it. So they're stuck ten miles from home without a bottle opener. Troy and Andy beg Wayne to go back for it. But he refuses as he says they will eat all the sandwiches.
After two hours, and after they have sworn on their tortoise lives that they will not eat the sandwiches, he finally agrees.
So Wayne sets off down the road at a steady pace.
20 days pass and he still isn't back and Troy and Andy are starving, but a promise is a promise.
Another 5 days and he still isn't back, but a promise is a promise.
Finally they can't take it any longer so they take out a sandwich each, and just as they are about to eat them, Wayne pops up from behind a rock and shouts, "I knew it! ... I'm not fucking going!"