Which of the following is true of the planet Earth?
The Earth revolves around the Sun.
The Earth is the third planet from the Sun.
The Earth is the only planet that is orbited by exactly one moon.
The Earth is the smallest planet in the solar system.

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.

Ashould be answer.