Exactly one

Not zero, not both. The rule that picks out "these two disagree". Build an XOR gate — and meet the gate every adder is hiding.

What you'll learn

• The XOR rule: output is on when the inputs differ.
• Why XOR is the "disagreement detector".
• How XOR secretly runs binary addition (you already saw it in lesson 5's half adder preview — this is a proper intro).

The idea

OR was "at least one". XOR is "exactly one": on when the inputs differ, off when they agree. Flip that around and you get XNOR — the equality check.

Why is XOR such a big deal?

• It's the Sum bit of binary addition: 0+0=0, 0+1=1, 1+0=1, and 1+1=0 (with a carry). That last case is where XOR earns its keep.
• It's the encryption primitive: XOR a bit with a key bit, and a second XOR with the same key recovers the original.
• It's the parity-check gate: XOR a bunch of inputs and you know whether an odd or even number were on.

The challenge

Wire two Levers (labeled Input A and Input B) into an XOR gate. Send the XOR's output to a Lamp labeled Output. Compare this truth table to the OR gate from lesson 3 — they disagree on exactly one row (both-inputs-on). That tiny difference is why XOR needs more internal gates than OR does.

Going deeper

In the redstone patterns library (/redstone/xor) you can see the actual Minecraft build — four torches, not two. XOR is the priciest of the two-input gates. Worth it.