When We Are Baptised, Why Pour Water On Our Heads?

Dear Maggie,
When people are baptized, why does the priest pour water on their head?
Baptismally Baffled
Dear Baptismally Baffled,
Me eight children were baptized with water flowin on their heads, so I think I got enough experience to answer yer question.  First, I should tell ya why we use water and not, say, sand or wine or dare I suggest, Scotch.  Baptism has long been a symbol of cleansing.  Even before Jesus came people would get baptized as a sign that they wanted to be cleansed of their sins.  The water also offers an image of birth.  It’s like we’re returning to the amniotic fluid and being born anew.  St. Paul added that going down into the water is like going into a grave and then coming out is like being resurrected.  Water is symbolically very rich indeed.
In the days of Jesus people were mostly dunked under the water in what is called immersion baptism.  Many Christian groups still practice immersion baptism and it is perfectly legitimate to do so. Some Episcopal Churches have immersion baptismal pools though I love a good river baptism myself. 
At first most Christians were adults converts, but as Christianity spread and continued people wanted to welcome their infants into the family of faith.  I don’t know if ya’ve ever tried dunking a baby under water but it will likely make the baby and the mother scream, so the church decided it would be just as good to pour water over the baby’s head.  This kind of baptism is called affusion and it goes back pretty far.  There is an early Christian document called the Didache (circ. 100 CE) that offers priests the guidance that while a river is preferable, or some other big body of water, three buckets over the head will do:
“…But if you have no living water, baptize into other water; and if you cannot do so in cold water, do so in warm. But if you have neither, pour out water three times upon the head into the name of Father and Son and Holy Spirit.”
There were a lot of Early Christians who were baptized in prison awaiting martyrdom and if you’ve been to most prisons you’ll know there aren’t any rivers flowin through ‘em.  Eventually the convenience of it caught on and affusion become very popular from the 10th century on, though there have always been those who prefer a good dunking.
I hope that resolves a bit of yer baptismal bafflement. 
Peace be to ya,