Why do Jews hang boxes containing prayers on their doorposts (mezuzahs)?
Mezzuzah
Common belief is that the boxes represent lamb's blood that the Jews placed on their doorposts during the tenth plague. The Pharaoh of Egypt refused to free the Israelites until the last plague. In order to force the Pharaoh to free the Israelites, God told Moses to tell Pharaoh that he intended to kill the first-born of both human and beast. To protect themselves, the Israelites were told to mark their houses with lamb's blood so that God could identify and "pass over" their homes. Some people believe the mezuzah symbolizes this event.

In fact, a mezuzah is the container that is placed on the doorpost to each room in a house, in order to be a constant reminder of God and the commandments. In the container is a prayer called the Shema. In the Shema, God tells us to remember God's words constantly, by writing them on the doorposts of the house and on the gates to our homes. The Shema is written on a small parchment, which goes into the container. Traditionally, the mezuzah is touched and the hand that touches it is kissed upon entering and leaving the room, as a further expression of love to God.