It works without return because the sample test (scroll a bit to see it in the coding page) tries to call factorial(). Talking about what he imported from math, factorial, it is imported as a function after all. Hope you get it.

Great.

It's not really that hard. Clear instructions:
You have a list of geese already made.
You just remove any item common in the geese list and the array to be filtered you get.
That's it?