Broken on impact Two competing models have been proposed for the adsorption of molecular hydrogen on platinum surfaces. Both invoke dissociation at surface defects but differ on whether hydrogen molecules diffuse along the surface before encountering a defect or adsorb only if they initially impact a defect site. Van Lent et al. studied the sticking of hydrogen molecules from a molecular beam scanned across a curved platinum single-crystal surface that varied in the density and type of defects exposed. Modeling of the results was consistent only with the second model invoking direct impact. Science , this issue p. 155