help simpel math

Buy yourself a box of "Cherios". First, take 10 out of the box. Make sure you count them one by one. Put those 10 in an empty bowl. Then take 9 out the box, and count them, one by one and add those to the bowl. Now you should have 10 + 9 in the bowl. Now count them ALL. Whatever number you get is the RIGHT answer for 10+9. OK, understood?

because these people don't know what they are talking about. 

here is a complicated explanation

10=5*2 9=3*3

5*3=15 3*2=6

15+6=21