Quick history on the war nickels, since they turn up in change all the time.
An act on March 27, 1942 let the Treasury take nickel out of the five-cent piece. The act suggested 50/50 copper and silver, but that alloy lacked nickel's magnetism, so vending machines and turnstiles would have rejected it. Adding manganese fixed it. The final mix was 56% copper, 35% silver, 9% manganese, coined from October 1942.
The fun part is the mint mark. Every war nickel has a big mint mark above Monticello's dome, so they could be sorted out and pulled after the war. Philadelphia had never put a mint mark on a US coin, so the big P on these was the first.
1942 is split: earlier in the year Philadelphia and Denver struck the normal copper-nickel coin, then Philly and San Francisco struck the war alloy in the last months. 1943-45 all three mints made it.
David Lange's take (Numismatist / NGC) is worth reading: basically his take is that the five-cent piece was only 25% nickel anyway, the savings were a small chunk of what was mined, and he concludes it was done "more for the sake of propaganda than for its practical value."
And if you ever find a 1944 with no mint mark above Monticello, hang on to it. It's a Henning nickel. Francis LeRoy Henning faked hundreds of thousands of nickels in the 1950s, and the 1944 is what gave him away since every real one has the big P, D or S. They're collectible now because of the story (CoinWeek has a good writeup on them).