does the attached lid create an air-tight seal?

No. Thankfully every Tubie comes with an attached lid, so it will never go missing. However, it doesn't create a water or air-tight seal. If you're filling them with yoghurt or smoothie mixture they won't leak out when closed or left on the side. However we do recommend propping them up slightly in the freezer so they sit upright (also means they freeze with a flat top). You can achieve this by putting them into a freezer-safe bowl or container.