When atoms or molecules collide, scientists expect them to bounce off one another like balls on a pool table. Not so when two carbon monoxide molecules meet in just the right way: they square dance instead. Using computer simulations, David H. Parker of Radboud University. and colleagues found that if the molecules meet carbon-to-carbon with a collision velocity of about 800 m/s, the two molecules will rotate in place, propelled by the oxygen atoms’ interaction as the molecules turn their “backs” to each other, before each continues on its original trajectory (Science 2020, DOI: 10.1126/science.aan2729). The motion looks quite similar to a dosi- do, in which square dancers circle each other back to back. The effect is to turn the molecules’ forward momentum into rotational energy. Parker says the behavior happens only in about 1 of 20 collisions, but existing models don’t account for it. In interstellar space, CO molecules are likely to emit that rotational energy as light, which is captured by the telescopes that produce images of dust clouds and nebulae. These collisions may be more common near comets and some exoplanets, which have very high CO concentrations. Parker and his colleagues are now looking for other molecular dance moves.