Virtual reality (VR) can provide access to otherwise inaccessible aspects of the world and thus promote science learning. We developed a VR learning tool about the water cycle, with 11 lessons for classroom teaching at the primary level. We assessed prior knowledge before a four-week intervention and learning outcomes directly after the…

Many experimenters, starting with Ernst Mach in 1883, have reported that if a device alternately sucks in and then expels a surrounding fluid, it moves in the same direction as if it only expelled fluid. This surprising phenomenon, which we call "Machian propulsion", is explained by conservation of momentum: the outflow efficiently transfers…