In one example, a structure for supporting a printhead includes: a substrate having a first side and a second side; multiple inlets each protruding from the first side of the substrate through which liquids may be introduced into the structure from the outlets of liquid containers that can be attached to the structure; multiple openings through the substrate from the first side to the second side, each opening positioned near one of the inlets at a location exposed to the outlet of a corresponding one of the containers when the container is attached to the structure; and an air channel on the second side of the substrate connecting each of one or more of the openings to another opening.