The lock and key concept is important for various biological and colloidal self-assembly processes. We study the basic and common underlying mechanisms within a model system which consists of a biaxial elliptical key immersed in a hard-sphere solvent and a smooth lock substrate into which the key can fit geometrically. If the rotational degrees of freedom of the key are constrained, the key faces on its way into the lock a free-energy barrier too high to be overcome by thermal fluctuations. However, if the key is unconstrained, the range of the effective attraction of the lock is significantly increased so that the key can, guided by a complex interplay of entropic forces and torques, successfully follow a trajectory into the lock which avoids the free-energy barrier.
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