Chemical reactions often proceed via highly unstable intermediates with lifetimes that are too short for techniques like X-ray crystallography to be usable. Now Takehide Kawamichi and colleagues of the University of Tokyo have found a solution. Their technique uses the tiny spaces in highly porous materials as molecular flasks, which slow the reactions due to steric (space restriction) effects. Cooling to cryogenic temperatures and reheating can effectively start and stop reactions and opens up the possibility of making movies of reactions. Using a reaction between an amine and an aldehyde the team observed the intermediate hemiaminal and determined its structure.