The marshmallow test described here will hardly qualify as a valid scientific experiment. Passing the test won't indicate your kid is on the fast track to Harvard. That said, it is a fun activity to do with your kids, and an opportunity to impart a valuable lesson in patience.
Step 1: The Set-Up
1. Let him/her sit in the chair.
2. Tell them, "If you don't eat that marshmallow in 5 min. I will give you something to go with that marshmallow"(another marshmallow, chocolate, candy, etc.)
3. Once they get that message, leave the room.
That guy below this was really... TEMPTED!
Step 2: The Experiment
Return and reward the child with an extra marshmallow if deserved. Allow them to now eat both marshmallows. Explain that you are proud of them for being patient. If they ate the marshmallow while you were gone, try to use the moment for teaching about delayed gratification.
If you want to laugh somemore keep rewarding them with more marshmallows the longer they wait.
Step 3: The Aftermath
Please ignore the last part, I don't know why i kept recording.