If you zap me 21 sats...
...and then I zap you 21 sats...
...we both leave the interaction with no more sats than we entered it, but we both gain something from it.

https://image.nostr.build/768d6433ab6d4eae9ca8abcb116c686287b6b5d7267d728f8e5bfcd977a5d86f.jpg