Demand Reflection

The demand term that I am reflecting on is Compliments. In class we have talked about compliments as a good or service that is in conjunction with another good or service. In my life outside of school I encountered this at home. When I was getting ready to hangout with some friends last night I realized I was really hungry and that a peanut butter and jelly sounded amazing. However when I walked to the fridge to see if we had peanut butter, jelly, and bread; I soon found  out that we had lots or peanut butter and lots of jelly, but no bread!!! It caused me to thing of what I could make it with instead and it didn't work with anything else in the house so i couldn't have my PB&J because we didn't have the compliment of bread to make me a sandwich. 

My question would be why is it that somethings only work with one thing like PB&J on bread? It tastes funny otherwise...