The jar was cylindrical and there's an equation for the volume of a cylinder. The volume of a cylinder is given by:
V = Base Area*Height = pi*r^2*h
I told them to count the number of Legos high, and they each did. Then I asked them to count the number of Legos across (diameter) and they both did. We plugged the numbers in a calculator and they both had a number they wrote down and put in the guess box. A week later after the contest had ended, I received a phone call letting me know that my 4 year old had the closest guess! She was only 1 Lego off!!!
So who said knowing math doesn't come with perks? Okay, perks for my 4 year old, but perks nonetheless!