The appalachian mountains formed as a result of

Converging plate boundaries. continent to continent plate boundary converging if you want to be more specific.
to converge means to bring something together, that's how I always remembered. Transform boundaries move side to side, and are the reason for mainly fault lines, such as the San Andreas Fault in California. All plate boundaries do occur at subduction zones.

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.