Four numbers, one of which does not belong with the others. These free printable worksheets ask which one and leave space underneath for the reason, because the reason is the part worth having.
If they get stuck
The answer key prints on a separate page.
The property is never stated. It might be that three are even and one is odd, three are multiples of ten and one is not, three are two-digit and one is three-digit. A child has to look for what the three share before they can say what the fourth is missing, and that search is a different mental move from anything a page of problems asks for.
Written answers are what make it work. "25" is a guess that can be right by accident; "25, because the others are all even" cannot. There is room on every question for a sentence, and marking the sentence rather than the circled number is what turns the page from a puzzle into practice.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Odd one out, step by step