How do subsitutes affect demand?
Substitutes are goods that can be used in place of one another. If the price of a substitute goes up, say tape, then the demand for the other good, say glue, will increase. If the price of tape were to decrease, then we would see the demand of glue fall. It is important to note that the demand of tape is not changing, people are simply buying a greater/lesser quantity at that higher price. With glue, however, we will see that people are willing to buy a greater or lesser amount at every single price.