An Incidental Question

Just wondering-

What's the difference between casserole and salad?

Is a casserole just a cooked salad?

Here's an interesting situation to ponder-

A family finishes their dinner, a casserole. There is some left over, which they put in the refrigerator. After a night in the refrigerator, it gets cold. Is it now salad? If so, than why go through all the fuss of making up a word for each? Shouldn't hot salad or cold casserole suffice? How do linguists sleep at night?