Secret levels are found within regular levels, usually hidden in non-obvious places or behind decorations. The tell-tale sign of a secret level is the presence of strange, fly-like creatures, shown in the image to the right.
Entering a secret level will cause the narrator to announce that you've found a secret and you'll be transported to the level via a screen transition.
Secret levels are very easy to complete, and it is almost impossible to earn any mark less than A++. However, when a player goes through the secret level instead of completing the level that transported them there, they will get a C regardless of how well they did.