Most seven-year-olds find science fascinating, especially if it involves a project that seems to work like magic, or one that allows them to make a mess. Encourage a love for science in your child or classroom by performing experiments that are not only simple and fun, but also introduce important principals of science in a way that is not intimidating.
Demonstrate how density works with a sink-or-float experiment. Fill a bowl with standard tap water. Hold up a raw egg and ask the students if they think the egg will float. After taking predictions, place the egg in the water. It will sink. Tell the students that the egg is denser than the water, which means the water can't hold it up. Next, explain that you can add stuff to the water to make it denser than the egg. Add some salt to the water (do this beforehand so you know how much to add -- approximately six tablespoons per one glass of water). Carefully place the egg in the salted water and it will now float. Tell the students that the salt makes the water have more "particles" so it becomes denser and holds up the egg. Young children may not understand the difference between density and weight, so explain that the water is "stronger," not heavier, than the egg. Obviously this is very simplistic but it will help the students begin to differentiate between weight and density.
Chemistry is Cool
Show the students that some materials act differently when combined. This is somewhat messy science, but the children will love it. Pour a bit of water and a bit of cornflour into separate bowls. Let the students touch and describe each material. List the adjectives on the board as the students describe what they are seeing and feeling. Then add some of the water to the cornstarch. Continue adding until you have a fairly thick paste. Let the students play with it and describe how it behaves. Note that it acts like both a liquid and a solid. Tell them that this is a type of polymer, which means that it is made up of parts (or introduce them to the concept of molecules, if you wish) that link together and stretch. Polymers are not quite solid and not quite liquid, but something in-between. Talk about other polymers, such as rubber bands and even gum.