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This is a very broad question @thatgirlcaleigh . I'm taking micro economics right now, so maybe I can help a little...
There are a lot of determinants to both the supply and demand. Demand could be elastic or inelastic also. So changes in prices can flex, but depending on the type of good on the market and the market demand itself will vary.
If price of corn goes up because supply of large crops gets wiped out from a swarm of locus, then the price of corn will go up at the grocery store. Demand will go down for the corn because it is not a household staple and the cost becomes too much for the consumer when they could simply by a substitute vegetable like carrots instead..
Basically, if price goes down it is often because supply is up. If price is low, there is often more of a demand for that item. But that is a very general assumption.
...I'm attaching my instructors notes for this section of my class, maybe they can be of some help for you.