You can help your child explore the idea of whole and parts of a whole whenever you cut or break up something into pieces.
Try cutting your child's sandwich in half. Notice how the two pieces fit together to make the same size sandwich? What if you cut it into four pieces? Try cutting her sandwich into different creative shapes, each the same size.
Try cutting it into eight triangles and invite her to put it back together. Notice with her that the more pieces there are, the smaller each piece is.