To accomplish the first plague, the Plague of Blood, God tells Moses to instruct Aaron to stretch the staff over the waters.
It is so strange that Aaron, as opposed to Moses, was chosen to be the messenger to carry out this plague.
The Medrash explains: When Moses was a baby, he was saved by the waters of the Nile. It would be an act of ingratitude for Moses to now cause an affliction to the very waters that saved him. Aaron was therefore chosen for this task.
So hidden in the story is a great lesson for Moses and for all of us. It is called “reciprocal kindness” or in language of our Sages, “hakarat hatov”, meaning recognising the good.
When you benefit from an act of kindness shown to you by another, you have an obligation to recognise and return the favour. Besides being an act of human decency, reciprocal kindness ensures that the amount of kindness in the world is always doubled.
The custom of a guest giving a gift to a host is a wonderful application of this principle.
May we always do kindnesses and recognise the kindnesses that are done for us.