Why are rain forests important to the world?
species found in the rain forest have been used for medical purposes.
rain forests are the natural habitat for 30 percent of the world's plant and animal species.
rain forests are a valuable food source for the world.
rain forests increase greenhouse gases.

The correct answer is rain forests increase greenhouse gases. I know for sure it's not the first or third answer. Rain forests are the habitats for many animal and plants but I'm pretty sure its over 50% not 30%. 

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.