Logic GatesChoose a gate for every slot until the whole truth table ticks.
One wire into a two-input gate? Then both of its inputs get that same signal. AND and OR pass it straight through, NAND and NOR flip it like NOT, and XOR always gives 0, because its two inputs can never differ.
Reading a truth table: each row is one way the switches can be set. Two switches give 4 rows, three give 8 and four give 16: the number doubles with every switch.
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NOT has only one input, so it only fits a slot with one wire going in. Put a two-input gate in a one-wire slot and both of its inputs get the same signal: AND and OR pass it through, NAND and NOR flip it, and XOR always gives 0. Every level has been checked by computer, so every one can be solved. Nothing is saved: ticks for solved levels last only this visit.
Boolean logic from GCSE Computer Science and beyond: the six gates and their truth tables, combining gates, De Morgan's laws, building any gate from NAND, and real circuits such as half and full adders, a multiplexer, a parity check and a comparator.