A scene can sense when the viewer's cursor is over a shape, has touched a shape, or is dragging atop a shape.
A scene can also trigger animations on a viewer's touch, enabling the viewer to move and rotate shapes.
There are four main action sensor types:
TouchSensor senses touch,
SphereSensor senses drags in a spherical way,
CylinderSensor senses drags cylindrically, and
PlaneSensor senses drags around a plane.
These X3D scenes are adapted directly from the
original VRML 2.0 Sourcebook chapter examples.
Introduction to VRML97 SIGGRAPH98 course notes.