Dry cold cereal is a great tool for teaching kids about shapes. Fill a baggie with several types of cereal and let your toddler sort them out by shape.
Squares: Chex, Quaker Oat Squares
There are more shapes in some sugary cereals (Lucky Charms comes to mind). If you don’t want to buy an entire box, consider buying the little one-serving packs and letting your kids glue the shapes onto paper. Or just eating a small serving for a special treat.
And of course, there are other foods that come in basic shapes. Some crackers are triangle or rectangle shaped. When you start looking around your kitchen, you may find little teaching opportunities everywhere!