Looking for an abstract concept that kids will have a hard time understanding? Try diffusion. Your kids might be able to rattle off its definition -- the passive movement of particles across a concentration gradient from a region of higher concentration to one of lower concentration -- but to truly understand it, they'll need some hands-on demonstrations.
Paper Wad Modeling
Gather some neighborhood kids together to demonstrate the difference between diffusion in hot and cold water-- and help your child let off steam at the same time! Divide the room in half with a line of tape on the floor, and position half of the kids in each section. The kids on one side of the room start off with three pink papers each, wadded up into balls, and the kids on the other side of the room do the same with green papers. Then direct them to throw the paper wads slowly for one minute (symbolizing diffusion in cold water), before counting how many wads of each color have made it to the opposite half of the room. Repeat while they throw the paper wads quickly for one minute (symbolizing diffusion in hot water). Kinesthetic learners will love this physical enactment of diffusion.
Diffusion Through Water
Take a look at this fast way to demonstrate how diffusion works through water. Prepare two cups of water -- one hot and one cold. Then add one drop of food coloring to each cup at the same time. Which one does your child think will spread first? Evaluate your child's hypothesis based on your results.
Diffusion Through Air
It may be tougher for your child to grasp the concept of diffusion through air -- which actually follows the same principles as diffusion through water. Just use a funnel to slide a small amount of vanilla extract into each of two balloons, and then blow up the balloons. Then put the balloons into two different (closed) boxes, one in the sun and one in an air conditioned room. Can your child guess which box will smell more like the extract? Can she figure out what on earth that has to do with rates of diffusion?
So your child's wondering why she should care about diffusion? Challenge her to research the relationship between diffusion and respiration in different organisms. Sure, oxygen molecules diffuse from our alveoli into our blood vessels, but many non-mammals use diffusion differently. For example, respiration and diffusion are related differently in regards to single-celled organisms, amphibians, echinoderms and fish. Create posters together displaying the information that your child has discovered.