~What can go through~...?

[swangeralexes]

Only some things are allowed through the green glass door, and some things are not allowed. It is your job to work out what is allowed through by guessing, followed by someone who knows and understands the rule/trick to the riddle telling you if you can take that which you guessed through the door.

(Please, if you know this game already, don't give it away)

Here are two clues to start you off:

1. What can get through, can't, but what cannot get through, can.
2. Behind the door you may attend classes at school or college but not courses at a university.

Good Luck!

So start asking if things can go through.