Turn sand from your next beach vacation into this easy-to-use clay, then build a permanent, air-hardened sand castle for a keepsake.
Add the sand, cornstarch, and alum to a saucepan and use a wooden spoon to combine them completely.
Add the water and stir the mixture until it is smooth and the cornstarch is dissolved.
Cook the clay over medium-low heat, stirring occasionally. The clay should start to thicken within a minute or two. Stir more vigorously, being sure to scrape the mixture from the sides and bottom of the pan.
After about 3 minutes of cooking, the clay will have thickened to the consistency of play dough. Remove the clay from the pan. Once it's cool enough to touch, you can start sculpting. (For best results, use the clay within a day or two of mixing it.)
Create your sand castle with towers and crenellated walls. We added a shell door and flag (a shell glued to a twig). The clay should dry completely within a week. If any pieces fall off after the clay dries, reattach them with tacky glue.