Airdrops have become increasingly popular as a way to incentivize early adopters and build hype around a new project.

Many startups will distribute tokens or coins to users free of charge in order to generate interest and drive adoption. However, airdrops can also be used as a tool for retention after a project has launched. Retroactive airdrops, in particular, are worth keeping an eye out for because they provide a way to earn "passive income" from your holdings.

Essentially, a retroactive airdrop is when a project decides to distribute tokens or coins to all holders of a specific NFT, regardless of whether they are the person who originally acquired it. Any future airdrops will be coming to the address that holds that NFT. That's why retroactive NFTs are sometimes called NFT keys.

Similarly, if you held Bitcoin at the time of the Bitcoin Cash fork, you would have been entitled to receive BCH. In both cases, simply holding the original asset would have resulted in receiving additional coins - no action on your part was required. Of course, retroactive airdrops are not always well publicized and it can be difficult to know when they are happening in advance. However, they are worth keeping an eye out for as they provide a way to earn "passive income" from your cryptocurrency holdings.