Want to bond with a new group of people? You could cover all your clothing in glue, or, if you're on a budget, you could just laugh with them. If you've ever been the only person in a group who didn't get a joke, you probably already know that laughter is critically important to successful social interactions. Scientists have found that mutual laughter helps people feel at ease around each other, and laughing when others are laughing makes those others more likely to accept you into their group. What's more, research suggests that laughter really may be the best medicine; laughter provides a vigorous workout to tighten your tummy and strengthen your heart, and regular laughing may boost your immune system. Fortunately, just about everybody can laugh. Just in case you need some pointers, though, you've come to the right place.

[edit] Steps

Think of something you find funny. Not surprisingly, the easiest way to laugh is to think of something that you personally find very amusing. It seems a no-brainer, but it can be useful for those occasions where everybody but you is laughing at a joke. Why would you want to laugh just because others are laughing? Laughing along with one or more people shows them that you're intelligent enough to understand what has been said—and, more basically, that you were actually paying attention to what was said. It also demonstrates that you're not a dour, unpleasant person, and it builds a common experience. Laughter is a form of body language, and like other forms of body language, it isn't easy to fake, so when you want to appear genuinely amused it's best to actually be amused, even if you have to amuse yourself.
Smile. If you do want to fake... er, create a laugh, start with a smile. Scientists have found that genuine laughter is almost accompanied by the contraction of about 15 facial muscles, most of which are the same you use when you smile. Remember to smile with your eyes as well as your mouth. Smiling not only is a part of the natural laugh reflex; it can actually put you in a better mood and make you more apt to laugh.
Laugh at the appropriate time. For reasons not yet understood, genuine laughs almost always begin at the end of a phrase or sentence. That is, they do not interrupt spoken phrases, but rather punctuate speech when the speaker would normally pause to breathe or start a new thought. You have probably noticed that stand-up comedians, for instance, pause at certain times during or after their jokes. These are spaces for laughter, and if a comedian had the lung capacity and the audacity to deliver a two-minute monologue without ever pausing, it's quite possible no one would laugh, regardless of how funny the routine was.
Match your vowels. A typical laugh consists of short vocalizations, each with the same vowel sound. So, for example, "ha ha ha" will sound like a normal laugh, as will "bo ho ho," but "ha ho ha" just sounds weird. And kind of scary.
Time your vocalizations. Research shows that the individual vocalizations that make up a "natural" laugh are about 210 milliseconds apart. If your watch has a millisecond hand, this should be fairly easy to practice. If you're off a millisecond or two, don't worry, just practice harder. What, no millisecond hand? Well, you'll just have to wing it.
Feel it in your belly. You've got your timing and vowels right, but if you just say "ha ha ha" people are liable to think that you are either stuttering or mocking them. If this is not your desired effect, remember that sustained or frequent laughter is a proven way to strengthen and tone your abdominal muscles. The reason, of course, is that a good laugh uses those muscles to rather forcefully expel short bursts of air. Practice this by doing your best belly laugh—it doesn't necessarily need to be loud—for one minute. You will feel the burn. You may want to be seated if you try this, however, as laughter is characterized by irregular breathing and is actually similar to gasping for air.
Taper your laugh. The typical laugh starts relatively loud and then tapers gradually off. While this isn't always the case, laughs that increase in volume or that stop abruptly are generally suspicious.

[edit] Tips

While laughter is most often associated with humor, people laugh for a variety of reasons. Many researchers believe that fear or nervousness, for example, are at the root of almost all laughter.
Get together with some friends. Studies show that people laugh far more when they are with other people than when they are alone, and laughter flows most freely when people feel comfortable with each other. Indeed, laughter is contagious.
If you find that you frequently don't "get" jokes, or if you have trouble finding humor in everyday situations, maybe you just need to pay better attention.

[edit] Warnings

While scientists believe that laughter has a whole host of health benefits, it's not recommended for some people, such as those who have recently had certain surgeries or those with certain medical conditions. Always follow the advice of your medical professional, and if you ever experience pain or discomfort while laughing, especially if the discomfort persists after you have stopped laughing, consult your doctor immediately.
As is the case with other forms of body language, it's difficult to fake laughter, and if you're unconvincing, people may suspect there's something phony about you.
Too much "laughter" may cause people to believe that you have serious mental problems.