18. Reza has a glass ball filled with water that contains tiny plants and shrimp that eat those plants. No material can get in or out, but light can get through the glass when it is placed in sunlight
The number of energy storage molecules in the plants and shrimp started out high, but then the
glass ball was moved, and the number of energy storage molecules decreased. Was the glass ball
moved into the light or into the dark? What happened to the amount of carbon in the water?

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.