A man took his two  sons , ages seven and  four to the playground at their 
local park. The seven year old was very proud as he was able to read the sign 
with all the rules to his brother.

"Do not jump on the merry-go-round when in motion."

"Go down the slide while sitting only."

"Only one child on a swing at a time."

There were about twenty rules and the boys promised to obey them all, if he 
would trust them and let them play without Daddy standing by. They said that 
they were too old to be watched and their friends would tease them calling 
them babies if Dad stayed. He made them promise to be good and obey the rules, 
and rejoined his wife preparing our picnic lunch.

When it was time to get the children, he decided to watch them at a distance 
for a while to see how reliable they were in following his instructions. He 
found that they obeyed most of the printed instructions. That is, all but one. 
They would get on the tall semicircular slide and go down head first or 
backward.

Angrily, he picked up the children and took them over to the posted 
regulations and made the seven year old read it aloud again. Then he asked
the boys what they had to say for themselves.

The four year old answered immediately, "Don't be silly, Dad. No one uses a 
slide rule anymore!"