Some of the best toys you can buy for preschoolers aren't toys. When young children are given the opportunity to explore, manipulate and construct their own knowledge they learn and retain that knowledge from what they have experienced. Water is one of the best mediums to use to promote learning in young children, and "It's Free!".  Water allows children to experiment with the concepts of liquid versus solid (ice cubes), comparison of quanities, flow, suction and vacume- just to name a few.  By giving children a supply of water and a few tools that you may have lying around such as measuring cups; different size and shape clear containers such as ketchup bottles, bear shaped honey bottles, milk jugs, mayonaise jars;  funnels; turkey baisters; sponges; clear tubing such as airline tubing found in aquariums; egg beaters; straws young children have the items they need to learn many basic scientific and math concepts. Those fancy waterways with locks systems are neat but a better option may be to make your own. Give your child the a chance to learn those same concepts by constructing their own waterways system with rain gutters, blocks, plastic tubing, and spend the money on a small child sizes suction pump.