Need ideas for Math fair

Hello guys, i need some help with ideas for a math fair. I'm looking for a interesting and uncommon math question or puzzle with solution involved math,.
So if you have any math puzzle feel free to share with me, i appreciate your help XD.

I actually have this one math question that i would like to do in the math fair but i cant remember what the steps are i only remember the solution.
Well the following steps are what i roughly remember and hopefully someone will tell me the correct steps

step 1: pick 2 numbers between 10-99 and then multiply them together, have your friend doing the same thing but with different numbers
step 2: multiply the answers you both got
step 3: after you multiply both answers, you hide one number from the answer then you tell the rest of the numbers
and have someone guess the number you hided

solution: each pair add up to 9 (what it mean is 1+8=9, 2+7=9 3+6=9 4+5=9 9+0=9)

Run the Count Buffon needle experiment. It consists of participants throwing a needle or a wooden stick onto a sheet of paper or card which has a series of parallel lines drawn onto it. The probability of the needle or stick intersecting a line is a fraction involving pi. By counting the fraction of intersections, as more and more needles/sticks are thrown, an approximation to pi should be obtained. We did this at my former college years ago,. Perversely, the more results we got, the further we moved from pi !

Sounds cool, i will share this information with my group and see what they think about it.