The $29.99 all-metal desktop Stirling engine kit is one of the simplest. It can be assembled in 15 minutes and is recommended for ages 14 and up. Also for $29.99 is the Exploring Kid Steam Engine Kit.

Now $60, down $40 off its usual $100 sticker price, this miniature engine kit seems to suggest otherwise. Get It! Made specifically for educational purposes, this Playz kit is divided into over ...