In Drosophila antennae, most olfactory receptor neurons (ORNs) send axons to both sides of the brain. So how do fruitflies know which way to turn in response to an asymmetrically presented odour? The authors showed that the axon branch ipsilateral to the ORN soma fires earlier and faster than the contralateral branch. Thus, an odour that stimulates one antenna more strongly than another results in faster and stronger activation of projection neurons downstream from the ORN branch ipsilateral to that antenna, which explains the flies' ability to lateralize odours.