On those hot summer days when even the snakes stay inside, it can be good to have a constructive indoor activity to do. If you have none, or, like me, don't want to do any of the ones you already have, why not make a soroban? This one is made from cardboard, bamboo skewers, electrical tape, plastic beads and string. It only took about five minutes to make, but I've already spent over an hour playing with it. I am really into math, and there is quite a bit of information available online about both basic operations and advanced calculations. I couldn't delve as deep as I would have liked into the amazing world of the abacus with this soroban because of its limited size, but I plan to shop around on ebay for a real one soon.