hiding sermon notes

I'm reading Debby Applegate's The Most Famous Man in America: The Biography of Henry Ward Beecher, which just won the Pulitzer for biography.

Applegate says that Beecher, early in his career, used small pieces of paper, cut to fit into his Bible, so no one would know he was using notes when he preached. My father did the same thing. Dad used onion-skin paper; when his Bible was closed, the notes were invisible. I wonder if they taught that in seminary?