Cloud seeding is an attempt to modify the weather to produce precipitation. A cloud contains water droplets and ice crystals. Precipitation occurs when the ice crystals grow large enough to fall as rain, snow, hail, or other precipitation. Unless the conditions are right for ice crystals to grow, precipitation does not occur. Cloud seeding is an attempt to convert the super-cooled droplets of liquid water in a cloud to ice crystals. Dry ice (solid carbon dioxide) and silver iodide are the substances most often used to seed clouds to transform the water droplets to ice crystals. Seeding is most efficient when the seeding agent is dropped from an airplane on the top of a cloud.