Interesting Math for 3-graders I'm supposed to give a 30 minutes math lecture tomorrow at my 3-grade daughter's class. Can you give me some ideas of mathemathical puzzles, riddles, facts etc. that would interest kids at this age?
I'll go first - Gauss' formula for the sum of an arithmetic sequence.

Show them a pizza. Ask how many pieces they can make by cutting it once.

The answer should be 2. Now ask them for 2 cutting. The answer should be 4. Then ask them to try for 3 cuttings. I hope some of them will the correct answer, which is 7. Then explain them what is happening in each cutting, how the number of pieces are increasing and why. 
Then help them to formulate the problem for n cutting. The formula is
$C(n) = \frac{n(n+1)}{2} + 1$
If they understand everything so far, you can then extend the problem from pizza to cake :-)
Hope they will have fun! :-)

