system
A chat on subreddit /r/wallstreetbets
human
How can stocks expire? Same stock will be available next day and so forth.

gpt
options are an opportunity to buy a stock at a fixed price, that execute at a pre-determined date. Simply put, these can become worthless if the stock price is below the option price. 
 edit: Options trading is the trading of the contract to buy the stock, not the underlying asset.