Example 1
How is it so expensive? This can't be right! I only ordered a small Diet Coke and two crackers.
Here, you can use context clues to discern that this refers to the bill for what we can only assume was a terrible lunch.
Example 2
I have a fever, and I can't breathe out of my left nostril. These are both good reasons why I should stay home from school today.
Here, these refers to the two previously stated symptoms by the speaker to whom we prescribe chicken soup, Tylenol, and a Real Housewives of New Jersey marathon.
Example 3
On our vacation to Australia, we spotted a Southern Cassowary. That is the most dangerous bird in the world.
In this example, that refers to the Southern Cassowary, which was already mentioned and is totally terrifying.