For generations, a family lived on a mountain in a cabin, beside a crystal clear mountain stream. At any time they could walk out their front door and drink from the stream, but they never did. In the cabin was a pipe, running from the kitchen sink into the stream. Whenever a family member was thirsty they would go to the sink to drink.
But the pipe was so rusty and clogged, that when they turned on the tap drips of water would come out, with a weird taste. It never occurred to them to go out to the stream to drink directly, because generation after generation of the family living in the house had taught them to only use the tap.
In fact, because of this many of the younger family members doubted that the stream even existed, and many moved away, off the mountain.