A man needed a horse, so he went to a church and got one. Before he left, the priest told him that it was a special horse. In order to make the horse go, you say, "Thank God", and for it to stop you say, "Amen".
So the man left, and a few minutes later he dozed off on his horse. Hours later, he woke up and his horse was racing him towards the edge of a cliff. Just in time, he shouted "Amen!" and the horse stopped a few inches from the edge.
"Whew," said the man, "thank God!"