Educational Innovations Pit and Pendulum - For Science Classrooms, Zen Gardens, or Office Desktops - Replacement Sand Only MESMERIZING PATTERNS - This pendulum is hypnotic to watch as it traces beautiful Lissajous patterns in the sand base. EXPERIENCE PHYSICS  The pit and pendulum gives viewers a firsthand experience of math and physics. A Lissajous figure results from the two sine curves meeting at right angles. TEACHING FOR ALL GRADES - It can be used at upper levels to study harmonic motion as energy is transferred from a pendulum swinging in one plane to swinging in another plane. GREAT SCIENCE DECORATION - Great addition to any zen garden, school science table, and much more! The pit and pendulum is 21" tall! SAND BASE REPLACEMENT - Only the sand base is included in this purchase.