They then, on the Twits' return, have two birds drop some glue onto the couple's heads. Upon entering their house, the Twits presume that their living room has turned upside down and that they must therefore be standing on the ceiling, so they stand on their heads and become stuck.
When the Twits return, two birds place glue on the couple's heads. The Twits believe their living room has been turned upside down and that they must be standing on the ceiling; thus, they stand on their heads and become stuck.