Create a cake pan or dish to represent the whole cake, or a whole number. Use a real cake pan, a round baking sheet or make a cake pan or complete cake from cardboard.
Create pieces for the cake. For example, make two cake halves, four cake quarters, six cake sixths, eight cake eights, 10 10th cake slices, 12 12ths and 16 16ths. Label each piece of cake with the fraction. For example, each cake half would be labeled "1/2." Decorate these to make them look like cake slices or have students make their own.
Create flashcards or use existing flashcards that have math problems involving subtracting uncommon denominators. Make sure all the problems can be solved using the cake pieces. You may need to make even smaller cake pieces, such as 20 20ths, or remove these cards.
Divide children into groups of four. Give each child a small stack of flashcards and a cake manipulative with all the "cake pieces" the students will need to solve the problems. Ask students to work as a group to solve the problems.
Demonstrate how to use the manipulative. For example, solve the problem "1/2 minus 1/3." Take a half piece and a third piece of the cake and place it on the whole form. Instruct students that they will now need to find new pieces to evenly fill this same space. Demonstrate how you can use five of the "1/6" pieces to evenly fill in each of the larger pieces.
Explain that students must find the largest alternate pieces. Explain that in the example "1/2 minus 1/3," three "1/6" pieces equals three sixths and two "1/6" pieces equals two sixths. This can be written into the new problem of "3/6 minus 2/6" which equals an answer of 1/6.