Response to the following problem:
The amount of water in a lake may be increased by action of underground springs, by inflow from a river, and by rain. It may be decreased by various outflows and by evaporation.
(a) Comment on the question: How much rain is there in the lake?
(b) Comment on the question: How much water in the lake is due to rain?
(c) What concept is analogous to "rain in the lake"?