Back in pre-Breathalyzer days, drivers checked their reflex speeds by getting someone to drop a pound note past their open forefinger and thumb. If you missed it three times in succession you were not fit to drive.
To test this, get someone to hold a 12-inch ruler vertically, with the bottom just level with your finger and thumb, held an inch apart. When they unexpectedly drop the ruler, snap finger and thumb together to catch it.
Many people can catch it after it has dropped 7 inches or less. The formula: S= 1/2 at2 (where S= distance in feet, a=acceleration due to gravity - 32ft per second - and t=time) shows that total elapsed time is not half a second in this case, but one-fifth of a second. If it were half a second, you'd miss the ruler as it would have travelled four feet]