Approximately 300 years ago, people wrote letters to each other but often found it difficult to keep their thoughts private as there were no envelopes, passwords or security codes.
They would often fold the flat sheet of paper to become its own envelope, a technique called 'locking'.
Now, researchers from Queen Mary University in London have found a way of reading the letter without damaging it and opening its seal. They have used a highly sensitive X-ray scanner and computer algorithms to open it virtually.
The letter examined dates back to July 31, 1697.