Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A student mixed baking soda with vinegar and observed that the reaction was endothermic. When is a reaction endothermic?
when it requires heat to make it happen
An endothermic reaction absorbs heat. It occurs when the enthalpy change is positive.