Cut some onion into small pieces and separate each layer. Each layer of the onion contains a thin layer of cells that can easily (usually!) be peeled off the rest of the onion. Place the layer of cells onto a clean slide, make sure not to fold the onion skin over on itself. Place a drop of iodine on top of the onion skin and then a cover slip. View under the microscope. You will note regular, rectangular shape of the plant cells, the cell wall and the nucleus.
One question to ask your students…. They are plant cells, why are there no chloroplasts? [Think about where onions grow…]