In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant.

[Q]: The bat sends out sound waves from their mouth or nose. The sound waves hit an object. Echoes are produced. The echo returns to the bat's ears. The bat hears the echo. The bat can figure out where the object is located.
[A]: sound waves ,echo

[Q]: A bird lays an egg. The egg hatches into a baby bird. Baby bird eats. Grows into an adult bird. The bird finds a mate. The pair build a nest. The birds lay eggs.
[A]: egg ,baby bird ,adult bird ,nest ,eggs

[Q]: Coal is loaded into the firebox. The boiler produces steam under high pressure. The steam generated in the boiler flows down into a cylinder. Just ahead of the wheels, pushing a tight-fitting plunger, the piston. The piston is connected to one or more of the locomotive's wheels through a kind of arm-elbow-shoulder joint called a crank and connecting rod. As the piston pushes, the crank and connecting rod turn the locomotive's wheels and power the train along. An outlet valve opens and the piston pushes the steam back through the cylinder and out up the locomotive's chimney.
[A]:
coal ,steam