Here's a life tip for you - if a stranger ever comes up to you in the street, telling you he'll give you a dollar bill for free, and all you have to do is catch it between your fingers when he drops it, get the hell out of there, it's a trap!
Seriously though, test it out with a friend, and unless you're an exceptional human being, you will never ever catch that dollar bill. But don't feel bad - there's a mathematical reason for why it can't be done, and it's rooted in the biological fact that the reaction time for most humans is about 0.2 seconds.
Yep, 0.2 seconds approximately how long it takes for you to see something, process it, and physically react to it, which is actually pretty impressive when you think about it. But that's not nearly quick enough for your eyes to see that falling dollar bill, and have your brain to process that information, and instruct your fingers to snap together in time to catch it.
So case closed, right? Well not quite, because you're perfectly capable of catching things on the fly - humans aren't completely useless - so why not some sweet, sweet cash?
As the Numberphile video above explains, it all comes down to some good old high school maths. We all know that when we drop an item from a height, you can figure out the time it will take to get to the ground based on the falling velocity and the distance travelled. Or you can figured out the distance travelled based on the falling velocity and time it takes to get to the ground.
You just need to use an equation like this, D = 1/2 gt2, where D = distance, t = time, g = gravititation acceleration, and gt = falling velocity. On Earth, gravitational acceleration is measured at 9.8 m/s2, but for the purposes of this calculation, Numberphile mathematician Tadashi Tokieda has rounded that value up to 10.
If all that is giving you maths homework nightmares, don't worry, the video above does all the work. Just sit back, relax, and feel smart by osmosis.
So when you use this equation to figure out a distance based on the human reaction time of 0.2 seconds and gravitational acceleration value of 10, you get 0.2 metres, or 20 cm.
If you measure a dollar bill, you'll find that it's around 15 cm long, which means that if someone is holding it at 5 cm or less above your fingers, it's pretty much impossible for you to react fast enough to catch it. Curse you, slow mortal brain!
But don't give up just yet, because there is hope if your one dream in life is to catch falling money between your fingers.
You'll have to watch the Numberphile video above to find out why even someone without above average reaction times could improve their chances of achieving this feat, but let's just say if you do agree to that stranger's bet in the street, make sure he lets you choose which fingers to use.