And as for why we put a mezuzah on our doorposts...
Deuteronomy 6:5-6:9
"You shall love the Lord your G-d with all your heart, with all your soul, and with all your might. Set these words which I command you this day upon your heart. Teach them faithfully to your children; speak of them in your home and on your way; when you lie down and when you rise up. Bind them as a sign upon your hand; let them be a symbol before your eyes; inscribe them upon the doorposts of your house and on your gates.
Beautiful. :o)
Thanks.