How is it possible that if I combine 100 Ohm resistors in different ways, I get _LESS_ resistance than original?
When you add resistors in parallel, you add additional paths for the current to flow, so it is easier for current to flow through the two resistors in parallel, than it is for it to flow through a single resistor.

Do you know Ohm's Law?  E = I x R, where E is voltage (electromotive force)(V is often used insead of E, and some languages use U), I is current in Amps, and R is resistance in Ohms.







simulate this circuit &ndash; Schematic created using CircuitLab

In the above circuit, Ohm's Law says 10 mA will flow through each resistor, so the total current produced bythe power supply will be 20 mA - twice the current for a single resistor.

Again using Ohm's Law, turned around to R = E/I = 10/.02 = 500 ohms. so the two 1000 ohm resistors in parallel act like a single 500 Ohm resistor.