It's a simple idea we learn from our early childhood: We laugh when we're happy and cry when we're sad.
But sometimes it's not that simple. What happens when you laugh at a video of someone falling off a ladder, or find yourself straining to avoid hysterics when a home video shows a son hitting a line drive into his father's groin?
All of this points to a simple conclusion: Pain makes us laugh.
The ancient Greeks knew it, 17th century philosopher Thomas Hobbes knew it when he wrote "Leviathan," Chevy Chase knew it when he made a name for himself portraying a bumbling version of President Gerald Ford, and Johnny Knoxville knew it when he turned self-injury into a successful MTV television show called "Jackass."
Pain isn't supposed to be funny, yet it is a staple of humor and prompts laughter from audiences.
"There's sort of a universal element to slapstick," explained Diana Mahony, a psychologist and humor researcher with Brigham Young University and the author of "God Made Us to Laugh."
But she draws a distinction between the slapstick of Bugs Bunny or "Saturday Night Live" and laughing at a painful video shown on YouTube or "America's Funniest Home Videos."
"There's a lot of aggression and ill will in certain types of humor," said Mahony, noting that, despite humor's positive connotation, it isn't always beneficial. "The stuff that's going on right now, I think, is just a reflection of some of the negative aspects of human nature."
She points to theories from ancient Greece and Hobbes to explain why some find pain funny -- because it can make the person laughing feel greater than the object of his or her derision.
Mahony explained the mindset as, "I laugh in triumph and superiority at the foibles and stupidity of other people."
One example of this is the Darwin Awards, a Web site that recounts the exploits of people (a few of the stories are real) who, through poor decision-making or a seeming lack of common sense, remove themselves from the gene pool.
But Mahony notes that if your own relative or friend made a mistake that put them on the list, you would likely search for a way to justify their actions.
Another possible explanation for the humor is the detachment most people feel from the person injured on TV or in an Internet video. In addition to a person they probably don't know personally, the detachment can stem from the situation where the person gets hurt, which is often somewhat outlandish -- like an absurd skateboarding stunt.
"Whatever pain we see is just one component of what is otherwise a funny circumstance," explained Dr. Emanuel Maidenberg, a psychiatrist at UCLA Medical School.
The context, he said, delivers a mood of humor, which can prompt the audience to follow suit.
He noted that while people may laugh, the person being hurt is typically pained by something they chose to do. Television shows in the United States do not broadcast torture, he noted, a different situation where a person who found it humorous could very well have a deeper psychological issue.
While plenty of people laugh at situations that are not supposed to be humorous -- during a moment of silence, for example -- our laughing at them doesn't necessarily mean we find it funny.
"The common misconception about laughter is that laughter is, for the most part, a response to humor," Mahony said.