On an astronaut's way to Mars, they will be in a space for 3 years. Their bones will start to become very fragile. So, I created a reflex hammer so astronuaunts can check their reflexes and make sure that they are not too slow. Having slow reflexes means that you do not work out enough, and not working out in space can be very dangerous. It can completely slow the mission down, and cause them to be out in outer space for much longer than needed. Each person would have their own. With four or five people on one mission, they would need to have a bunch for each time they use it. It would be much easier just to print it out on a 3D printer and then recycle it after every use to make other things. The average reaction time is 215 milliseconds, so once it gets much longer than that, it could start getting very unhealthy. The reflex hammer is ½ inch wide, 1 ⅛ inch high, and 5 ⅞ inches long. This is made out of plastic, so it is one hundred percent printable.