system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: short selling stocks and currencies

I've read a bit about it but I would really value a more simple and I depth explanation. Thanks in advance!
gpt
Say you think your local store is going to have a sale on Star Wars DVDs.  
You want to make some money.  
You got a friend who can lend you some Star Wars DVDs.  
You take those Star Wars DVDs and sell them on eBay.
Now the time comes when you have to return your DVDs, you go down to the store where a sale is going on, you buy back the same DVDs at a discount and you return them to your friend.  
You pocket the difference that is the discount.  
Alternatively situation: You reach the store and find they're sold out! The only one selling Star Wars DVDs is Neckebard's Super Pricey Nerd DVDs, they charge 20x times what you sold them on eBay for. Though luck, you have to buy it at that price and return it to your friend.    
  
Basically, borrow the stock, sell now, buy back later, return the stock.    
Short selling is considered risky because there's limited profit (the company goes bankrupt and the stock is worthless, you keep the entire amount you sold it for), but the risk is unlimited ( the company announces they have a product that cures cancer, world hunger, and the impulse to commit terrorism, they get a controlling share of the world, you sold for $10, the stock is now $90 bajillion, you're effectively bankrupt).