A well-made educational toy can do a lot to foster curious young minds. But what if you're not sure where to start shopping, or wonder which toys are appropriate at your kid's age? Amazon is taking an unusual step to help out. It just launched a STEM Club that delivers one pre-selected science, tech, engineering or math toy to your door each month in exchange for a $20 subscription fee. The internet retailer gives you a choice of age ranges (3-4, 5-7 and 8-13), and promises to pick only the best toys from "top trusted brands." Shipping is free, too.