A childhood belief about lies

When my brother was four we visited a pond by my mother's workplace that ducks, and though I had been a many times before, for him it was the first time. Me and my siblings told him that we had been looking forward to this day for a long time and then explained that we had found him in this pond as a baby and that the Ducks were his real family and that when we left we were leaving him behind to be with them. At first he tried to argue that it couldn't be but we solemnly assured him it was all true. He totally believed it, dead serious and stared out into the lake and really thought about it and decided that being with his duck family was the right thing to do. When mom found out about our prank she quickly cleared everything up but he still acted weird for a while whenever he saw ducks.