[rahimnathwani]

I remember one day in primary school a teacher had us play a game where we had to propose things to take on vacation. People would call out a suggestion, and the teacher would say yes or no.

The aim of the game was to figure out the rule. She said no to 'novel' and 'money', but yes to 'book' and 'currency'.

[overlordalex]

Spoiler: Is it the number of letters? That is, only an even-length string is permitted?