The disappearing-volts divider
Measure a divider before and after adding a load.
Difficulty: Breadboard. Estimated build time: about 20 minutes. 3-line bill of materials. Compare supplier offers when available. Build a 5 V divider and make its output sag on purpose.
Prepare the bench
Use an isolated, regulated 5 V source with a 20 mA current limit and a digital voltmeter with approximately 10 MΩ input resistance. These are bench tools, not included in the shopping list. Disconnect power before changing connections. Confirm breadboard rail continuity and check for supply shorts before powering up.
Make the unloaded divider
Connect R1 = 10 kΩ from +5 V to node OUT, and R2 = 10 kΩ from OUT to ground. Put the voltmeter in DC volts, black lead at ground and red lead at OUT. Expect approximately 2.50 V, with small differences from tolerance and meter loading.
Add a hungry neighbour
Power off. Add RL = 10 kΩ from OUT to ground, in parallel with R2. Power on: R2 || RL is 5 kΩ, so OUT should be about 1.67 V. The load did not change the source voltage; it changed the divider ratio.
Predict, then compare
Remove RL with power off and confirm the output recovers. Calculate loaded and unloaded values before measuring. If the result is near 0 V or 5 V, inspect the shared OUT node and split breadboard rails. Never place the current-range meter across OUT.