After penning beloved children’s stories such as The Cat in the Hat and Yertle the Turtle, Theodore Geisel, better known as Dr. Seuss, was bet $50 by Bennett Cerf, co-founder of Random House, that he couldn’t write a new book using only 50 unique words.

Seuss (sidenote: his family says it’s pronounced “zoice”) took the bet and produced Green Eggs and Ham, which tells the story of an unnamed narrator who’s pestered to try an oddly colored breakfast by the annoyingly persistent Sam-I-Am.

Cerf never paid Seuss the $50 he owed him. However, Green Eggs and Ham, which was published on August 12, 1960, went on to sell over 15 million copies in North America alone.

Green Eggs and Ham wasn’t the first time someone challenged Seuss to use a limited number of words in a story. Seuss wrote The Cat and the Hat after William Spaulding, an educator trying to find a perfect book to teach first graders vocabulary words, challenged Seuss to use only 225 words from a list of 348 vocabulary words. The final product took nine months to complete and contained 236 unique words, only 11 words over the limit.

Seuss was a strong believer that short, simple stories were one of the best ways to educate future generations. As of 2014, his books have sold over 600 million copies worldwide.