Gather the kids around! Using household materials, you can make "play-dough" (not to be confused with the trademarked kids' stuff) and learn a bit about science while you're at it!

Designed for kids aged 4-10, the dough-making process is quick, not too finicky about measurements, and produces a totally reasonable toy, suitable for stamping and molding. You can also add food coloring to enhance the fun factor. Now go find a child (with permission) and enjoy this lesson on chemistry:

For more on this activity, check out The Royal Institution's ExpeRimental Play Dough page, which includes a helpful info sheet and certificates of achievement for kids who complete the activity and learn a little chemistry in the process.