Fill a clean glass jar with water. Use a sturdy jar that easily supports the weight of the potato.
Push three or four toothpicks in a circle around the middle of a potato. Suspend the potato over the jar with the toothpicks resting on the jar's rim. Half of the potato stays above rim of the jar. The toothpicks prevent the potato from falling into the jar and allow the bottom of the potato to touch water.
Place the jar in a dark place and check the water level often. The end of the potato must stay submerged in water. Roots will appear from the bottom of the potato in approximately two to three weeks. Sprouts will also begin to grow from the top of the potato.
Move the potato plant to a warm, sunny location. Keep the jar filled with clean water as needed. If the potato plant becomes long and leggy with few leaves, cut back the sprouts to three or four inches tall. This encourages the potato plant to produce additional sprouts from the top, resulting in a bushy plant.