What is the difference between stimuli and stimulus?

The word “stimuli” is the plural form of the word “stimulus”, so they are both the same. A piece of meat would be a stimulus for a dog; a piece of meat and a horn blowing at the same time would, together, be stimuli. I believe it is derived from the Latin.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.