Children are naturally curious and have many questions about the natural phenomena they observe. Use these questions as starting points to introduce scientific concepts. For example, a child's question about why he should brush his teeth provides a gateway to teach about germs and disease. When explaining any concept, ask open-ended questions rather than simply lecturing. Give children enough time to come up with an answer, rephrasing the question if necessary, or providing a few hints. This approach ignites the spirit of scientific inquiry in those little minds that will prove useful later in life, too.
It is important to select age-appropriate topics that are easy for children to grasp. You can teach preschoolers, for example, about combining two colors to yield a third, different color, but you would find it difficult to get them to understand the component colors of white light. When explaining science to young children, base your teaching on what they already know from experience, and seek to build upon that knowledge base. An easy way to do this is to conduct a classroom quiz to identify what children already know, and prepare lesson plans accordingly.
Whenever possible, use concrete materials or manipulatives to explain science topics. Provide children with opportunities for hands-on experience to improve their grasp of the subject. For example, let the class plant seeds in a pot to learn about germination and plant growth. Point out everyday occurrences that exemplify a science concept and encourage children to try a few simple experiments on their own. Ask them to observe what happens when a Popsicle is exposed to sunlight, or contrast the behavior of a coin with that of a plastic bead when dropped into a bucket of water.
Children have a short attention span and find repetition boring. Vary the techniques you use to teach science so as to hold their interest. Read a short science-related article on an age-appropriate and interesting topic, hold a science quiz and discuss the life of a well-known scientist. Play science DVDs, take a field trip to a zoo or a science museum and invite a local college chemistry teacher to demonstrate a few "magic" experiments. Whichever activity you use, make sure to use the right scientific vocabulary without watering down the concept.