After a few busy days of exploration using "thinker toys", the students have come to recognize that odd and even numbers follow each other in a pattern.
They are also able to recognize that it is the digit in the ones place of a two digit number that will determine whether the number is odd or even.
If a second car is added filled with cubes, first we do NOT need to count them as we know that there are 10 in the car and that 10 add 3 is 13. Second, we know that 10 is an even number so we can put our attention to the digit in the ones place of the two digit number. Again, 13 is an odd number because one cube does not have a partner.
This reasoning follows for all numbers where the digit in the ones place is 3. Above is the concrete example of 23, which is an odd number, but we know that the same would be true is the two digit number was 53, or 83 or 43.