Wason's 2-4-6 task

Consider this triplet of numbers: 2, 4, 6.

I'm thinking of a rule that that triplet adheres to. I want you to guess the rule.

To do this, you can give me another triplet, and I'll tell you whether it adheres to the rule or not.

You should test as many triplets as you need to in order to figure out the rule.

This is based on an experiment Peter Wason did in 1960. This page is part of a review of the book Thinking 101.