Key

Keys have appeared in Crash Bandicoot, Crash Team Racing,  Crash Nitro Kart and Crash: Mind Over Mutant

Crash Bandicoot
Crash gets keys in the Dr. Neo Cortex bonus rounds. Once Crash reaches the end Cortex drops a key and Crash can use it to get into a locked level. There's two of these bonus rounds and each are found in Sunset Vista and Jaws Of Darkness. The keys unlock Whole Hog which is on the second island and Fumbling In The Dark which is on the third. This is the only time up to Crash: Mind Over Mutant that keys have appeared in the main series. This game has only two keys.

Crash Team Racing
In this racing game keys serve a slightly different purpose than in Crash 1 and now they are part of the storyline. The player has to get the keys from Ripper Roo, Papu Papu, Komodo Joe and Pinstripe Potoroo by racing each of them and winning their keys in order to race the alien Nitros Oxide. Then once the players beat them they give the player the key to the next door which leads to the next world. With the exception of Ripper Roo they almost always give a warning that Oxide won't be as easy. (Although Ripper Roo's unused soundclips in which he talks in normal English, he does warn the player about Oxide). So the player uses keys to open doors to other worlds. This game has four keys.

Crash Nitro Kart
The keys in CNK hasn't really changed and has the same purpose as CTR had. They're still part of the storyline and the player needs to race against each of Emperor Velo the 27th's galatic champions Krunk, Nash, Norm and Geary for each of their keys to become the galatic champion. Keys also open gates which lead to different worlds. Like CTR this game has four keys.

Crash: Mind Over Mutant
Keys also appear Mind Over Mutant they are used for things such as opening doors to new levels in the Ratcicle Kingdom and Wasteland.