You can use a stem from a tomato plant to generate a whole new tomato plant and if you're patient enough, a tomato farm. ;) The cells in tomato stems are able to become roots. Simply cut off a stem from a full tomato plant, clipping off any flowers or buds. Make a hole in some damp potting soil in a container and plant the stem. Keep in a warm yet shady area and keep moist, gradually allowing them to have more sun. This process should take a couple weeks. You can then transplant them into a larger pot or into the ground. They also make fabulous gifts!
Great to know this. No need to buy many plants.