Unfortunately, you cannot bookmark the cards at the moment. We will be adding that functionality in the future. However, as of now, the idea of Stack is to have the sites and apps that you visit most frequently always open and organized efficiently into Stacks and Spaces. Therefore, having them on the sidebar always open technically serves as a bookmark. Anyway, we are working on bookmarks and we will be incorporating a totally different bookmark functionality in the future. Below is the feature request for that. If you want to be notified when we release it, please upvote using your e-mail.

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