This one? (Just googled for "smartie" and "experiment".) Apparently a 3-year-old, after finding out that a Smarties tube actually contains pencils, will immediately believe that his mother also knows it contains pencils. Whereas a 6-year-old will be able to comprehend the concept that he might know something his mother doesn't, and will therefore realise that unless his mother has also checked carefully she will still think the tube contains Smarties.