Well, there's the one Dad used to tell:
An engineer had two horses, and could never tell them apart. They were the same size, answered to each other's names, and were identical in every way. For years, he struggled to figure out a way to differentiate them... and then, one day, he burst into the house with a smile on his face. He'd finally done it! He tells his wife: "The white one's tail is two inches longer than the black one's!"
And one from Reader's Digest some years back:
A man leaving a movie theater stops another patron, who had her dog with her.
"Excuse me," he said,"but I couldn't help noticing how your dog seemed to be watching the movie. He wagged his tail in the happy parts, hung his head in the sad parts, fidgeted during the dull parts... he really seemed to enjoy it!"
"Yeah, I'm surprised, too," said the woman,"because he hated the book!"
Another from RD:
A cowboy walks into a bar and gets a beer. No sooner had he sat down than a man bursts in.
"Joe! Joe! Your horse's been stolen!"
The cowboy jumps up... and stops.
Wait a minute, he thinks, I walked into town. Can't be my horse!
So he sits back down. A few minutes later, another man bursts into the bar.
"Joe! Joe! There's a fire at the ranch! Come quick!"
The cowboy jumps up... and stops again.
Wait a minute... I live in the next town over. Can't be my ranch!
He sits back down - but then a third person bursts into the bar.
"Joe! Joe! It's your pa, he's sick!"
The cowboy jumps up... and stops.
Wait a minute, he thinks, my name isn't Joe!