Appropriate grades or age groups:
Elementary School
Middle School

Project instructions or description of technique:
Everyday life usually presents many opportunities for children (and adults!) to build their math skills. What you can use depends on the age of your child, but in general - show your child how to record some transactions in your checkbook, help you calculate the tip in a restaurant, figure how much a sale item would cost, decide if you have enough cash on hand for McDonald's, follow a recipe, figure gas mileage on your car ... the possibilities are endless.

Other suggestions or comments:
My daughter dreaded the chapter on percentages in her textbook, but relaxed when I pointed out that she could calculate 25% off of clothes and shoes faster than any adult I knew.