Havil is a delightful mathematician (author of "Gamma") who revels in the wonders of 18th century mathematics, and shares these with his readers in lively prose, and full algebraic derivations. He has done it again, combining newer results (Don E KNuth Elevator problem, Besicovitch set) with old ones (probability of a run of a certain length in digits in a random sequence) and a clever parlor/classroom trick based on that.