Thats a reference to a relatively famous linguistics study lol. A bunch of children were given questions such as "This is a man who knows how to gling. He is glinging. He did the same thing yesterday. Yesterday he _____." or presenting them with a wug and then asking what the plural of wug is. The children had very consistent answers to these nonsense grammar questions, showing that grammar rules are mainly learned through experience instead of being memorized for each word.