vintage postcard
If you were living in Ancient Babylon (six thousand years ago) you might bake a letter along with your daily bread.
Messages were written on clay tablets, which were then baked to harden them. The tablets were then covered with more clay and popped in the oven a second time.
The inner tablet could only be read by breaking open the outer layer of clay. The 'secret' message was thus secure. A hammer is my kind of letter opener!