Food web only contains interactions between trophic levels while an interaction web shows both trophic and non-trophic interactions.
I believe it would be the leaves because that helps collect water! :)
You can classify them in what phase they are in: solid, liquid, gas
You can classify them in Sizes
You can classify them by the number of moons they have
You can classify them by distance from the sun
You can classify them all as heliocentric or orbiting/circling around the sun
You can classify them by color