I recently volunteered to help out at my daughter's school for their Earth Day celebration. I didn't know what I was in for, but it worked out great. It turns out, that the station they had me at was right up my alley. I would be helping kids plant tomatoes in self watering pots made from used 20oz water bottles. I thought it was such a great idea, I wanted to share it with you. I think these would be best for starting small plants and seedlings...although I think this could easily be used with a 2 Liter soda bottle as well.
Cut your water/soda bottle(s) in half. You can make your top half (the part you drink from) as long as you want, but it will need to sit without tipping over in the bottom half.
Plant your seed or seedling in the top half of your bottle. Having the cap is not important as the opening at the mouth of the bottle will water your plant.
Fill the bottom half of your bottle with water and place the top half (mouth down) so it sits in the bottom half. Put it in a sunny spot and watch it grow! Replant as needed.