If an Easter egg is akin to finding that one last After Eight in a box full of empty wrappers, then finding a whole game inside a game is like finding there's another tray at the bottom of a presumed empty box.
There's something so inherently cool about a developer including a hidden video game inside the one you're already playing.
More often that not, it's usually in the form of an earlier game in a series, a sort of "Hey, look how far we've come" kind of acknowledgement. Or, it's something the studio has worked on as a side project, that occasionally goes on to be its own published game.
Sometimes it's nice to take a break from saving the world from the legions of Hell to play an old game... about saving Mars from the legions of Hell. Maybe your knuckles hurt from taking on rival street gangs, so you put your skills to the test on some arcade cabinets instead.
Whatever the reason, coming across or unlocking one of these hidden gems is a great way to spruce up the fun in a game. If there's an inherent challenge or reward to doing so, it's a nice little caveat for those aiming to 100% their games too.
So let's take a trip down memory lane with some games... in games.
10. Ninja Gaiden (2004) - Ninja Gaiden 1/2/3
It wouldn't be too much to say that Ninja Gaiden really hit its stride with 2004's reboot. Forget your "like Dark Souls" tagline, this was the original controller breaker.
A strict and unforgiving action game like many hadn't seen before, outside the likes of Devil May Cry and God of War, Ninja Gaiden had a point to prove: get good.
Paying homage to the unforgiving platformers that in turn gave life to this also unforgiving franchise, you can unlock the first three Ninja Gaiden games to get your nostalgia fix.
And in keeping with the hard as nails theme, they aren't easy to unlock.
To start with, you have to collect fifty of the hidden Scarabs dotted about, which is no easy feat. That unlocks the first game. The other two require some fiddly platforming and precision aiming, which sounds pretty easy compared to the scavenger hunt.
But by the time you unlock them all, it feels like self-flagellation: do you want to continue the difficult main game, or stir up some retroactive rage with the early games?
Hmm, tough call...