Invisible gases are a form of matter that have volume or occupy space. In this activity, water flows down through two identical funnels, each inserted in a two-hole stopper, into two identical flasks. The water flows at very different rates because a clay plug has been hidden in one of the holes in one two-hole stopper. In the slow flask some of the water actually shoots back out! The activity highlights the importance of teaching that catalyzes minds-on learning by activating students’ attention and challenging their prior knowledge. Engaging learners in “dialogue” with natural phenomena, one another, their own prior conceptions, and the teacher is a core principle of constructivist teaching.