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:
Stars burn mass. The star burns until it runs out of fuel. The star can no longer support its weight. The layers of hydrogen press down on the core. The core gets smaller and smaller under the pressure. The pressure causes gravity to get stronger. The gravity becomes so powerful that nothing can escape it. Anything that gets close to it, is drawn into it by its gravity. Light is drawn into it. Light can not escape. 
answer:
Mass ,Fuel ,Gravity 


question:
They take in fluids and liquids. They process the fluid. They take out the toxins. Produce urine. Urine is released. The nutrients are returned to the body. 
answer:
fluids and liquids ; fluid ; nutrients ,toxins ,urine 


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