To keep the puppets upright during filming they had to be tacked to the set through their conveniently black flipper-like feet. The tacks can often be seen appearing and disappearing as the Clangers move around, as can the consequent slight damage to the set.
The Clangers did not speak but they made a whistling noise (a Swanee whistle was used for this effect) which varied in pitch, loudness and rhythm. Many viewers tried to imagine words spoken with the same intonation. In episode Chicken (1969), some doors start to open and then jam half-way. Major Clanger climbs out of a burrow and the intonation of his whistles fits the words "Oh sod it! The bloody thing's stuck again." But this is all in the viewers' imaginations.
The first Smallfilms project to be made in colour.