Ok. Suppose you are on another planet. Suppose you just helped the aliens on that planet have a better more earth like atmosphere. But as you went to visit them you get off your space ship and you see something weird thats like green and covered in vegetation and you don't now if its alive or not? Its not moving so.. on your way back to your ship after a long day you see that there's more of it so you take some in a plastic container. You get home and you go up to your room. You then remember how to know if somethings alive or not! So you study it for a few days because you are on winter break. You see in your new microscope that it keeps breaking off and making a whole new one. So it reproduces and that means its alive so you check that one off your lists. Then you give it some water and it soaks it all up and seems to grow more. So it drinks and maybe eats check that one off your list. Then after a couple of weeks and then weeks turn into months you see that it began to change colors until it wouldn't reproduce and then it wouldn't eat. So it died was this thing once alive? So you go back to the planet and you tell them what had happened and then they tell you, "Yes it was.. because we have been studying your planet's vegetation also and we found some of the same results!" So you ended up finding out that that yucky green stuff was alive and now you know that it means to be alive and how you know if the thing you see is alive or maybe not.
Fire isn't alive, Rocks aren't alive But trees are even though they don't move.