Steve writes in with these build instructions for a classic bathtub playtoy, the tugboat. At first glance it seemed like a regular old wooden play boat, but there is something cooler here- it has a working steam engine!

I hadn’t heard of pop-pop boats before, but apparently they used to be common toys. The pop-pop motor is a phase-change heat engine (fancy name for a device that converts heat into kinetic energy), and the popping sound is caused when the water turns into steam and causes the metal cavity to expand. Here is a video of someone else’s build:

This looks like a pretty fun experiment- anyone make one of these before? Bonus points if it can be remote controlled.