Ohm's Law at the Bench: Picking the Resistor That Saves the LED

Build the LED and its resistor on a breadboard, then measure: the supply, the voltage across the LED, and the voltage across the resistor. They should add up to the supply. Then work out the current from the resistor's voltage and its measured resistance, and compare it with what you calculated. The LED's real forward voltage will not be exactly the table value — it varies by part, by colour bin, and with current — which is why the calculation is a starting point and the measurement is the answer. This also demonstrates Kirchhoff's voltage law without naming it: around any loop, the voltages sum to zero. Once you have seen it on a meter it stops being an abstraction and becomes a tool for finding where a missing volt went.