Question 23The edges of a regular tetrahedron with vertices $A,$ $B,$ $C,$ and $D$ each have length one. Find the least possible distance between a pair of points $P$ and $Q,$ where $P$ is on edge $AB$ and $Q$ is on edge $CD.$ $\frac12$$\frac34$$\frac{\sqrt2}{2}$$\frac{\sqrt3}{2}$$\frac{\sqrt3}{3}$
