The prostomium.
keeps it alive
it keeps the claw from getting caught on things.
It is the nature of the universe; the universe keeps getting older and so does everyone and everything in it. It's the nature of things.
Because the population keeps getting larger.
gravity keeps things in orbit
How about a fence around a yard? It does the same thing as a plasma membrane, it keeps the things that are supposed to stay in the yard, in the yard. And, it keeps the things that are outside the yard from getting into the yard. Every once in a while it lets a few select things either go in or out.
Cotton keeps most things warm.yarn keeps things warm to
the epiglottis
so you bagge keeps things in and storaged, what keeps things out and available?
Cell wall
Glass keeps beer colder due to its molecular structure.
Well, not everything, but most things like: Bugs, birds and other animals. And it keeps you from getting wet when I rains and...getting a sunburn. Dunno what else to list...