instruction:
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. 
question:
Fuels burns in the combustion chamber. The burning fuel creates energy. The upward motion cause air to be released into the cylinder. The piston compresses the air. A spark ignites the fuel and air inside the cylinder. The pressure pushes the piston down. The exhaust valve opens. The piston pushes the air out of the cylinder. 
answer:
fuel ,air ,spark ,pressure 


question:
Sediment becomes loose from the rest of the soil. The sediment rolls down hill towards a river. The heavy items sink to the bottom. The lighter items float on the top and in convection cycles. As the river flows the sediment is moved. The sediment gets dumped through tributaries into lakes and the ocean. The sediment keeps moving around the world in convection currents. 
answer:
sediment ,river ,convection currents 


question:
The growing season starts. Bark grows on a tree. The inside of the tree grows. The bark is pushed out around the tree trunk. The bark leaves a ring. The growing season slows down and ends. 
answer:

bark ,ring