1. Its physical properties change by an increase or decrease of things.
2. The chemical or genetic makeup of it, sometimes a type of instrument
(not music a tool in this term) to observe the chemicals making up what happened.
Example for 1: A larvae of a mosquito is a very tiny organisms found in dirty water like lakes, rivers, swamps etc (not slat). Over a certain time period they mature and have physical changes, called stages. These physical changes can be observed with the naked eye.
Egg to Mosquito.
Example for 2: When someone with natural Black hair bleaches their hair that's a physical observation you can see in the process, but to see what's really happening you need special equipment.
(Another example if the one above doesn't help)
When there's oxygen for an rat to breathe in, its blood cells are healthy and working properly giving it a red look. But if there's a shortage or no oxygen the blood cells will start dying out in a limited time. Slowly as there's no oxygen in the air for the rat to breathe its veins starts showing. That blue you see, well yes that's why whenever you see someone whose been dead for awhile they have hints of blue on them.
Hope this helps
