If you've got a kid, about 4-6 years old, go grab a treat they like, a marshmallow or cookie or something- and take them into the emptiest room in the house. Tell that kid that they will get the treat, but you have to go do something or other, and if they can go 15 minutes without eating it and it's still there when you get back- but only if it's still there- they'll get a second treat.
Now see what happens. Honor whatever result you get. Odds are about 2 out of 3 that that marshmallow won't be there anymore.
You now know a fair bit about how your child is going to do in life. What you're testing here is delayed gratification, and you're doing the same thing done in a 1972 experiment at Stanford by Walter Mischel.
I found that there is actually a TED talk about this, and TED talks are always good. Joachim de Posada, take it away.
You really do not want to see that marshmallow gone, because every time follow-up studies are done on former participants, the ones who resisted turn out worlds better than the ones who didn't. They were more successful, had better grades, and in some cases, it's been the difference between staying in school and dropping out. Seeking instant gratification as a kid means seeking it as an adult, and I'll leave it to you to fill in your favorite example of instant gratification that turns out sour later.
In fact, the difference can be seen in your brain, as shown here. Some of the ex-kids who either made it or ate the marshmallow almost immediately were re-tested as adults, with various faces substituted for the marshmallow. The instant-gratification group showed increased activity in the ventral striatum. On the crazy off-chance you've never heard of the ventral striatum, it's been correlated with things like addiction, OCD, and even schizophrenia.
If your kid ate the marshmallow, though, you don't need to start thinking doom and gloom, and even if the marshmallow is still there, don't rest on it. It's not set in stone. Recent years of follow-up studies have shown that the status of the marshmallow can change from year to year. If your kid ate the marshmallow, what you want to do is to make self-control a priority lesson. Sesame Street made it their lesson of the day back on June 3... which quickly led to Stephen Colbert taking the test.
Try not to be like Stephen.