Here's a bit of old-fashioned math fun, requiring just a pen and a piece of paper. Find a kid (or a coworker with nothing better do) whose birthday (day of the month) you know. Watch the under-three-minute video below, and make a grid. Tell said kid/coworker/boss/frenemy that your magic number grid will predict the date of their birth. Then as the power of arithmetic blows their mind, release a cloud of glitter (be careful to avoid the eyes). This will make you appear cool and powerful.
But seriously, this is neat stuff. In the video, Curiosity Show explains how to make a simple grid that, through addition, will "predict" an already-known number. If you're lucky, it will spur some discussion about how math works! Enjoy.