Here’s a really interesting mathematical explanation on how the “catch a dollar” trick works. You know the trick: a person holds a bill vertically and says you can keep the bill if you can catch the dollar with your fingers when it drops. You never catch it. It’s really hard! Why?
As Numberphile explains, the reaction time in most humans is about 0.2 seconds. That is, it takes about that much time for our eyes to tell our brains to tell our body to do something (in this case, catch the bill).
That doesn’t seem that long though, so why is still so hard to get the dollar?