Algodoo

Aims to demonstrate the workings of a potentiometer (voltage divider).

In the initial state R1 is very small compared to R2 since the tap of the poti (black wire) is on the far left. This means that only a small voltage drops at R1 and the remainder drops over R2 and the wheel (lamp) so it can spin (glow).
You can also take the perspective of the electrons: R2 is a big obstacle, so many electrons take the easier path through the tap.

Now carefully (!!) drag the black tap towards the right, while the simulation is still running.
--> Now R1 is big compared to R2. We get a huge voltage drop over R1 and the remainder is not enough to reliably turn the wheel.
Yet again you can take the perspective of the electrons: R2 is close to zero. The electrons will take the easiest path and therefore dont go through the tap and the wheel.