Well, the distinction is important here. In the fruits we've got seedless varieties of, the fruit forms first and then the seeds form within it; all that has to happen is for that process to be interrupted. But in stone fruits like cherries, the stone forms first and then the fruit forms around it; creating a stoneless stone fruit would require somehow dissolving the stone after it was no longer needed.

We do, in fact, have some varieties of seedless apples. They just aren't very popular. There are two main problems:

First, even if the apple is seedless, it's still going to have a core, and people still aren't going to want to eat the core. So making it seedless isn't super profitable. (This problem is even more pronounced in cherries: it's not the seed that people object to, but rather the stone around the seed.)

Second, apples are notoriously hard to breed. The children are nothing like their parents. Basically each tree we plant is a new roll of the dice. The odds that a mutant seedless apple will also have other desirable properties like "has at least a hint of sweetness" and "is not a crabapple" are low.

I think the Romans might have had a decent seedless apple at one point, but, if so, it went the way of the silphium.

Well, the first clue was the way in which damage to the brain can alter the functioning of the mind, whereas damage to things like the foot or even the heart do not.

Later, as we developed imaging techniques, we were able to see how thinking different kinds of thoughts would alter the brain's electrical activity and blood flow patterns, in ways that it doesn't alter electrical activity or blood flow in organs such as the liver or stomach.

Good answer. A comparison could be drawn with sweat. Sweat cools your body by evaporating: the energy to change it from liquid to gas has to come from somewhere. Similarly, salted ice cools the ice cream by melting: the energy to change it from solid to liquid has to come from somewhere.

Entropy is a measure of how many different (microscopic) states would meet a given (macroscopic) description. For example, suppose we have 64 pennies on a chessboard. There's exactly one way for them to meet the description "all on square a1": the first penny would have to be on a1, and so would the second penny, and so would the third penny, and so on. On the other hand, there are 64!≈10^(89) ways for them to meet the description "one penny per square": there are 64 different pennies that could be on square a1, and after that's chosen there are 63 different pennies that could be on square a2, and after that's chosen there are 62 different pennies that could be on square a3, and so on. So "one penny per square" is much higher-entropy than "all on square a1". Similarly, a universe where all the matter/energy is in one place ("Big Bang") is lower-entropy than a universe where the matter/energy is all spread out evenly ("heat death").

There's a weird sort-of-law of physics that says "the entropy of the universe always increases over time". It is, in fact, the only known difference between the future and the past: all other laws of physics work the same in reverse as they do when running forward.