Simple projects that are already assembled are usually best for younger children. As children in kindergarten through second grade will not have highly developed motor skills, choose a project that will be easy to assemble and easy to display.
One easy-to-do project is a potato clock. It takes two potatoes, two large nails, copper wire, three alligator clips and one low-wattage digital battery. You can easily find these items in a kit online which usually comes with a stand. Remove the battery from the battery compartment of the clock. Number a potato one and the second two using a marker. Insert one nail in each potato and place a short piece of copper wire into each potato as far away from the nail as possible. Place one alligator clip on the copper wire in the first one and connect to the positive terminal on the clock's battery compartment. Do the same with potato two, but this time wire it to the negative terminal. Use the third alligator clip to connect the copper wire with the nail and you now have a working battery!
Other easy science projects for very young children are the Christmas tree kits you can purchase online that grow crystals on cardboard by placing the cardboard in simple chemical gel. These two experiments are safe and easy.
Children in secondary school -- third through fifth grades -- are usually introduced to Egyptian history. If your child is particularly taken by Egyptian history, introduce the idea of reproducing mummification -- not with a dead body, of course -- but with a simple piece of fruit like an apple. Since the mummification process involved dehydration, dehydrating an apple will mimic the process without being overly messy or time consuming. This project is perfect for secondary age children as it will require them to mix their own solution to treat the apple. Take 1/2 apple, 1 Popsicle or craft stick, 1 medium-sized plastic bag that seals, and make a Natron solution out of 1/4 cup table salt, 1/2 cup sodium carbonate (washing powder bleach) and 1/2 cup baking soda. Place the apple in the solution and observe the results.
As students in junior high turn their attention to the world at large, they may become more environmentally aware and have a growing interest in helping to preserve the environment. One experiment they may want to try is the oil slick experiment, which they can use to teach their fellow students about the dangers of environmental hazards such as oil spills. The ingredients are simple and easy to assemble. You need yellow vegetable oil, a clear plastic or glass bowl, water, spoon, fishnet cheesecloth or gauze, a tablespoon, cotton balls and a polypropylene cloth. Adding oil to water in the bowl, have the student gauge which item is best at trapping the oil. Ask the student to examine and explore the best way of getting rid of the oil safely and cleanly.
When high schoolers are asked to assemble a science fair project, they might whine a bit and say that science is boring. Motivate high school students to participate by letting them try an experiment with an innovative gadget that may generate interest through its technological appeal. One great item is the Radica Mind Flex Duel, which pits the EGG readings of two opponents against one another. The two participants move a ball through an obstacle course racing to see which one wins. With a cost of around a hundred dollars, this isn't the least expensive science gadget available, but it is one that will attract a great amount of attention. Ask students to prepare a display of the science behind the game.