Background:
Different “thought experiments” dominate teaching approaches to weightlessness, reducing students’ opportunities for active learning, which should include observations, descriptions, explanations and predictions of real phenomena. There is a controversy related to conceptual definitions of weight and weightlessness.

Material and methods:
Research has two parts: (1) documental research on the presence of thought experiments and other elements of “pedagogical content knowledge” in physics textbooks, carried out with common method of text analysis; (2) experimental research on predictions related to mercury’s behavior in free fall. We used a home-made falling box with attached camera to get visual data.

Results:
We report another textbook controversy regarding the position of the person that weighs her/himself in freely-falling elevator and absence of attention to “relative existence” of gravitational field. That “relativity” may resolve definition controversy. Experimental results show that the mercury drop neither took expected spherical shape nor performed oscillatory motions predicted by theory. Behavior of mercury drop depends strongly on elastic features of the “elevator’s floor”.

Conclusions:
Teachers should encourage students to enrich active learning of weightlessness by thinking how to test experimentally the answers to some conceptual questions, (subclass of “thought experiments”). Two previously published studies show that such students' learning of weightlessness might be feasible.