No wifi. No accounts. No servers. Your note is encoded into a little melody of beeps and played out loud; any nearby device that's listening hears the tones and decodes them back into text.
Works phone ↔ laptop, browser ↔ browser, or with a Playdate running the Sound Mailbox toy.
1Open this page on two devices in the same room, or grab a Playdate.
2One side taps Listen; the other types a note and hits Send.
3The note pops out of the envelope on the listening device.