what's on the cap:
A very beautiful and unique way to immortalize something forever is to put it on the blockchain. The blockchain is the decentralized database behind Bitcoin, where all transactions are stored. You can also upload short sentences to the blockchain and then store them forever.
This can be done via the page: https://opreturnbot.com/

Create cap:
This page is from @OP_RETURN_Bot from which the sentences are retweeted. Under these tweets you have the option to @ReturnCapBot A cap is then created that carries exactly this message on the front of the cap. On the right side is the block time of the block into which the message was written.

Some already created return caps are here: