Underlying article in Nature: https://www.nature.com/articles/s41467-021-21326-w https://www.nature.com/articles/s41467-021-21326-w The letters being opened ar

Underlying article in Nature: https://www.nature.com/articles/s41467-021-21326-w https://www.nature.com/articles/s41467-021-21326-w The letters being opened are “locked” with a technique that was obviated by sealed envelopes, but was prevalent in the Renaissance and Middle Ages. It’s super intricate, though! Might be a fun thing to try for your next mailed letter :) https://en.m.wikipedia.org/wiki/Letterlocking https://en.m.wikipedia.org/wiki/Letterlocking