Directions:   Combine 1/2 cup water and 1 cup corn starch in an old pot.  You will notice that the mixture is difficult to stir.  This is "oobleck" -- a non-Newtonian fluid that becomes hard like a solid when you apply pressure by stirring, then relaxes and flows like a liquid when you release the pressure.  Try picking up a fistful and squeezing it in your hand -- then letting go.  It will feel like a solid rock when pressed, then ooze out through your fingers when you let go!

Now add the other 1/2 cup water and 2 cups of sand to the mixture and stir.  Heat on a stove over a low setting for about 5 minutes, being sure to stir constantly.  When corn starch is heated it thickens -- so your mixture will eventually go from being runny to being thick like a paste.  Allow the mixture to cool before working with it; when ready, it should feel like sandy play dough. 

Now you can start building your sand castle!  Spray your mold(s) with non-stick spray, then firmly press in your sand dough.  Once you have filled your mold, turn it over and pop out your molded object.  Decorate with sea shells and allow to dry. 

Science:  Be sure to use a magnifying glass to study the sand you are using.  Beach sand is made up primarily of tiny bits of rocks and minerals that have been carried down to the ocean by rivers and streams, then washed ashore by the waves.  Studying sand can tell you a lot about the geology of the area where the beach is located.  Light colored beaches are usually rich with quartz.  But beaches can also be black and red, depending on the surrounding rock formations.