They're called Fukubukuro - and at the start of each New Year, thousands of people in Japan queue up for hours to get their hands on them.
Fukubukuro - or lucky bags - are essentially mystery goodie bags that contain anything from clothes to food, depending on the store selling them.
They started out as a way for Japanese department stores to get rid of old stock at the start of the year, but now have become an annual craze nationwide.
Sales for Fukubukuro open every year on 1 January, and they're typically sold across the entire first week of January, or until they run out.
It's unclear how exactly the Fukubukuro originated - there are multiple stories told - but one version says they were sold as early the 1900s when department stores started cropping up in Japan.
The department stores, it is said, wanted to get rid of their stock before the start of the New Year and so decided to sell mystery bags filled with random leftover goods at a sizeable discount.
Today, department stores around Japan and even global chains like Starbucks and Armani have jumped on the trend.
Bags are now no longer filled with random leftovers, but are usually stacked with premium objects that would be significantly more expensive if bought separately.
Many queue up for hours in front of their favourite stores just to get their hands on a Fukubukuro, as they are usually sold only in limited amounts.
"[Buying a Fukubukuro] is a bit like gambling," Clark Lawton from Japan told the BBC.
"I've bought Fukuburos myself... it's the excitement of not knowing what's inside them, and also the chance at getting something cheaper than its actual cost."
Every year, people take to social media to post pictures of their Fukubukuro "haul".
"I'm so satisfied with this year's Fukubukuro!" says this Twitter user. "Best of all, the size is perfect."
But some who might not be as lucky with their Fukuburo haul have taken to selling it online, or swapping it amongst friends.