This is an old puzzle that comes in a variety of forms. I've adapted it here from a Martin Gardner version in his "Aha! Gotcha" volume:
6 students make reservations for a dinner at a popular pizza place. But at the last minute a 7th student decides to join them.
When the kids arrive the hostess immediately sees that there are 7 diners for her table set-up of 6. But she is a clever one. Thinking on her feet, she decides to seat the first student in chair #1 and then have his girlfriend (the second student) sit on his lap temporarily. Then the hostess can sit student #3 in the 2nd chair, student #4 in the 3rd chair, student #5 in the 4th chair, and finally student #6 goes into chair #5. Chair #6 is thus still leftover, and so of course the hostess can now move the original girlfriend to that seat. Waaah-laaaaah!!
Hope she gets a big tip... or, maybe not!?
Do you see the flaw in her method?
(For any who don't see through the flaw I'll wait 24 hrs. and explain the simple answer in the comments below... and then you can go "DOH!")
I'm sure the young lass (another child prodigy) reported on in this nice article can spot the catch in the math: