Crafting 3D paper pop-ups can yield complex, expressive geometries, and can help develop spatial reasoning skills. However, designing the cuts and folds is often a frustrating process due to the strict geometric constraints. Foldlings is a tool that assists in this exploratory process, allowing a user to draw lines and be guided in creating well-defined pop-ups. We simulate the pop-up in 3D, allowing users to quickly preview their design before printing or laser cutting. We created an iOS app written in Swift, and performed informal user tests with 23 undergraduate students.