# Stores and calculations

### Possible and not

You compare calculations of energy before and after a change, to see if it was possible or not. That's the core. You can do that using description, including snapshots for the beginning and end of the change. It's much harder to jump straight into the calculations.

### Calculations you can do

There are eight kinds of calculations you can do to figure out how much energy is shifted. Each kind represents energy shifted to or from a store

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A good description concentrates on identifying which of these eight kinds of calculation you hope to do. The names of these stores remind you what to calculate.

### Doing calculations

Here is how you can do the calculations. Every one includes a trade-off: make your choice and multiply up.