We have a tester of user interface design in our home: our four year-old son David. If he can figure out how to use a device, then it qualifies as having simple user interface design (and carrying value to a four year-old!). Doors, light switches, lamps, and the vacuum cleaner came first. Then the TV on/off switch. Next came the kitchen appliances: the door sensor in the fridge and dishwasher buttons. Finally, the mailbox — a little harder since it is nearly out of reach. Most recently, David has figured out our iPod. David has figured out how to replay the same song (Philadelphia Chickens) over and over again by pressing the back button.
Common among these items is a clear indication of where the buttons are. Consider equally important tasks that David has not yet conquered: the Input button on the TV, the Play button on the DVD player. Unlike the iPod and dishwasher, each is one of a string of equivalent buttons. The dishwasher and iPod, in contrast, clearly distinguish the importance of different buttons, making it easier to experiment and learn.