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.

Billed as the world’s first, the FX10 3D composite printer with the FX10 Metal Kit print engine from Markforged Holding Corp. is said to enable printing both metal filaments and composites with ...