There are some excellent resources to find science activities for pre-school classes online. The PBS website www.pbs.org has resources available for preK-12 in a variety of topics, including science and technology, such as links to lesson plans, activities, projects, and videos. Other websites such as www.reachoutmichigan.org/funexperiments/quick/preschool.html and atozteacherstuff.com and other similar websites found through Google searches also have some free pre-school science activity ideas and resources available.
Your local library may also have some books or videos with science-related activities for pre-schoolers and young children -- just ask the librarian or media center specialist to help you locate these materials.

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