Montessori Odd and Even Math activity (also called Cards and Counters) concretely teaches a child what number is odd and what number is even, a concept which can otherwise seem very abstract to a child.
First, place numbers randomly on the rug and ask your child to set numbers one through ten horizontally at the top of the rug. (You want to make sure that your child knows numbers one through ten before introducing the concept of odd and even.) Then ask your child to place counters under each number, corresponding to that number's quantity.
You can use traditional Montessori Numerals and Counters material (buy here) or you can make it yourself: you would need numbers 1-10 and 55 counters (use marbles, wooden dots, holiday-inspired small objects, etc). We are using fall-inspired maple leaves instead of traditional red wooden counters.
This presentation makes it very obvious for a child to see when a number has a counter without a pair/on its own: meaning that the number is odd. As opposed to, when all counter-leaves have a complete set of pairs - meaning that the number is even.
See here "❤️Odd & Even (Montessori 🔢 Math 101 🎥 Series 🎇)" a video-post where Adrian summarizes how to determine which number is odd and which one is even.
See here our Christmas Odd and Even lesson.
p.s. The Montessori math curriculum is generally introduced in the following order: (1) Number Rods (introduce at around two years of age), then (2) Sandpaper Numbers, (3) Spindle Box, (4) Numbers Memory Game, (5) followed by Odd and Even activity as shown above.