Answer
Mechanically-interlocked molecular structures, such as rotaxanes, are composed of two or more molecules that are interlocked in a way that prevents their separation without breaking covalent bonds. Rotaxanes consist of a dumbbell-shaped molecule (the axle) that is threaded through a ring-shaped molecule (the wheel). The wheel is then stoppered at both ends by bulky groups that prevent it from sliding off the axle.