One of the most powerful elements of the egg carton model is its ability to focus on numerator or denominator. How?
The denominator--the number of parts in the whole--is modeled by string or a pen line.
The numerator--the parts you have--is modeled by "eggs," shown by pen on paper or by tile, plastic eggs, or whatever you have handy.
Yesterday, my students looked at fraction addition using egg cartons. We used a Work Mat to demonstrate each fraction being added, as well as the total. By simply moving the string, we could consider all the possible common denominators.
My new product, Egg Carton Fraction Addition Task Cards, provides opportunities to practice adding fractions with unlike denominators, using an egg carton model for support. Check it out (introductory sale!) at: