With this simple circuit that is built with the 555 IC you can test human reaction time. You can find out how fast do you react by trying to catch a flashing LED.
How does the human reaction tester works?
When S1 is ON the circuit acts as a astable multivibrator and the LED is lit for about 0.1 second and is flashing every 1.5 seconds. Since the human reaction time is more than this (>100 ms) it will be quite difficult to catch the LED when is ON. If you press S2 within that 0.1 second period of time when the LED is ON then the discharging of C1 stops and the LED stays lit.
You can change the ON and OFF alternating periods by changing the values of R1 and R2 or C1 to suit your preferences.