When he consulted the librarian for assistance on a resource that would help him teach Missy, Sheldon made an offhanded remark his sister was so stupid she writes fan mail to ALF.
The reference came towards the beginning of Young Sheldon, as Sheldon tried experiments to see if he could make Missy a genius.
This led him to the library in search of a means to better educate his sister, but he was having trouble finding the proper book.