How false memories form is unclear. To begin to address this, Ramirez et al. aimed to create a false hippocampal memory in mice using optogenetics. They placed mice in a novel environment and labelled activated neurons in the dentate gyrus by the targeted expression of channelrhodopsin 2. The mice were then placed in a different environment and underwent fear conditioning while the neurons activated in the first environment were optogenetically reactivated. Subsequent placement of the mice into the first environment elicited a fear response, indicating that a false fear memory had been created.