What is an example of consumers setting the demand?

What the market/people demand. IE, it's winter time and people need shovels. So people will go to buy shovels. As a whole, lets say a store sells 25 shovels that day. That is the market demand. That's how many shovels people want. If the store had only 20 shovels to sell, there are still 25 people who want shovels, so the market demand is 25, yet the supply is only 20.