Consider if you will a simple key.
Nothing fancy, just your ordinary house key.
Is a key easy to use? It might seem so. On a sunny day, when a sixteen-year-old lad with sharp eyes comes home from soccer practice, puts his key in the lock and turns it, it sure seems usable enough.
But later that day, Dad comes home carrying two big bags of groceries in his arms. He has to fumble a bit before the key goes into the lock, and eventually he drops the left bag and oranges go rolling all over the front porch, to the delight of the neighbor's dog, who starts barking and yipping and running around and generally being a nuisance.
Then, when Grandpa comes to visit, his hands shake a little and it takes him almost a minute to ...