Why are daylight and nighttime of equal length on an equinox?

The length of daylight and nighttime are of equal length on an equinox because the center of the sun spends a roughly equal amount of time above and below the horizon at every location on the Earth.

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.