Guess My Rule

Can you figure out what the rule is here?

ie. Why do the YES cards go together and why do the NO cards not belong?

Last Thursday, I was visiting a 1st/2nd grade class. The students were playing Guess My Rule in small groups. One student would choose a secret "rule" and the other students would take turns trying to figure out out which picture cards do and don't fit that rule. The rule maker starts the group off by showing one card that does fit the rule (YES) and one card that does not fit the rule (NO). Then it's up to the players to reason about which other cards may or may not fit the rule. I was impressed that not only were students making choices and having to provide a rationale from their choices, but they were discovering that they could learn as much about what the rule might be from their wrong guesses as from their right ones. They also had to grapple with the question of "when can I say for certain that I know what the rule is, and when am I still guessing?"