Create a simple voltmeter that prints the detected voltage (0 - 5 V) to the Serial Terminal whenever a button is pressed. Construct a voltage divider and then measure its output voltage using your voltmeter.

For example, if I measured the voltage between two 330 Ω resistors connected in series from 5 V to GND, I should see something near 2.5 V printed whenever I pressed the button on the breadboard: