Let's say you've got a stray cat in your neighborhood.
You've seen the cat scrounging around your house and your neighbor's house for food. You feel sorry for the cat and, one day, you decide to leave a little bowl of milk on the back porch. You watch dutifully from your kitchen window as the cat cautiously makes its way to the milk and laps it up.
The next day, you forget to put milk out, but the cat still comes around. You feed it again. The cat keeps coming back, every day, until you can't get it to leave.