Answer:
When energy is absorbed!
Step-by-step explanation:
Endo -> Within
Whenever you hear the suffix endo just think within! Endothermic reactions are when you are inputing energy for the reaction to proceed.
Exothermic reactions are when eneregy is being released this can be in many forms and one of the most common exothermic reactions is the breakage of the terminal phosphate on an ATP molecule which is extremely energetic (or exothermic)