My latest greatest math tool find. Draw a number line with sharpie on a ziploc slider bag and use the slider as the tool to add or subtract two numbers. I just filled the bags with addition and subtration cards. I put a piece of white paper in the bag also because it made the numbers a little easier to see. You can add a self check component by writing the answer on the back of each card. I probably should have started with zero...but you can use whatever numbers you want or leave it an open ended number line.