A detailed experimental and theoretical analysis of the alignment of grafted DNA molecules by a moving meniscus is presented. The existence and extent of the stretching (up to 2.14 times the unstretched length) depends critically on the properties of the surface. Molecules grafted at both ends exhibit a looplike shape which is scale invariant. An elastic model of this process, which we have called molecular combing, is introduced which (a) yields the extension force on various surfaces, (b) yields a value for the tensile strength of DNA, 476+/-84 pN, and (c) describes the shape of the loops with no fitting parameters.