Figure 1 (image) Kanazawa University Caption (a) Structure of a crown ether (18-crown-6), which can capture a metal ion in the cavity. (b) A bottle with a cork. We cannot put in and take out an object inside the bottle when tightly closed, but we can do so when a loose stopper is used. (c) Metallohost with caps. (i) The host can capture a guest cation in the central crown-ether-like cavity, and (ii) the caps (anions) can be introduced above and below the cavity. Credit Kanazawa University Usage Restrictions The image may only be used with appropriate caption and credit. Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.