Close up of the rubber strips (the gasket) with one the holes being held by a pair of tweezers. (You have to punch out the holes yourself. The strips then plug into the tops of the Lego pieces.)
https://s3-eu-central-1.amazonaws.com/mastodon-aral/media_attachments/files/110/889/816/068/196/221/original/ca9a7b5dc338fd7d.jpeg