You and a guest are sitting in a restaurant and decide to order some wine. The waiter comes over, pops the cork, and pours some for you. But, you notice how one of two things happens: they either show you the cork or leave it behind on your table. Is this a special gift from the restaurant? Do you give it back? What's the deal?
There actually is a reason behind it.
The gesture can be traced back to the 1800s where labels on wine bottles would wear off in the cellars. When patrons ordered wine back then, they had no way of telling if they were getting what they ordered. Those who purchased wine for their bars wouldn't have a way of knowing either. So, companies decided to brand the wine cork with their logo. Smart, huh?
But, okay. That was back in the 1800s. Why does this incident still occur today? Well, you wouldn't appreciate it very much if you ordered a nice bottle of wine only to discover it wasn't what it seemed. Wine corks are still shown to restaurant-goers today to guarantee you're getting what you asked.
Additionally, if you pinch the end of the cork you'll be able to gain some information. If the end of the cork is moist, you'll know your wine hasn't been oxidized. Essentially, wine bottles stopped with corks are placed on their sides. The corks expand in the neck of the bottle to prevent too much oxygen from seeping into the bottle, diminishing the wine's quality. If the cork becomes too dry, it'll let oxygen in easier; storing wine on its side prevents the cork from drying. So, give it a small pinch to see if it's a bit damp.
As for why waiters may leave the cork behind, that really boils down to improperly trained staff. Technically, the staff isn't supposed to leave the cork behind willy-nilly. Some restaurants will ask if you'd like the cork as a token of your night or if you'd rather leave it behind. Otherwise, it shouldn't be left without an explanation. You can always stop them before they go and politely request they take it back with them.
There you have it! This bizarre little ritual dates way back to the 1800s and is still practiced today. The corks are meant to provide more information into the wine you purchased. If the cork has the same branding as the label, you know you're not being duped. If you're really picky about your wine, check to see the moisture on the cork. The last thing you want is to dump money on a bottle that has oxidized wine inside. And here people thought there was no purpose to seeing the cork!