The Wise Old Owl sat in an oak,
The more he saw, the less he spoke,
The less he spoke, the more he heard,
Why can't we all be like that wise old bird?
After doing a little googling, I found that it was also used during World War Two (above) to discourage soldiers from divulging state secrets (along the lines of 'loose lips sink ships' and all that).
Do you remember any rhymes or stories like this from your childhood?