Be careful what sorts of bottles or jars you use. Be mindful of the environment that you get them from. A discarded bottle (i.e. one you find on the side of the road) may be toxic, poisonous, or otherwise harmful to you. Always exercise prudence and caution with waste materials. Be sure to clean reused materials thoroughly, and sanitize anything the bottle or jar may have come into contact with, including yourself.
Do not place the bottle in full sun. This miniature ecosystem can heat up too quickly and burn the plants or your fingers! (Don't leave it in the dark all the time either.)