Molecular repurposing was a method of reconfiguring matter into new forms. It was most effective on silicates, being able to turn rocks and sand into food.
The Slitheen family of the 347th century had molecular repurposing devices built into their bracelets. When they started a time travelling tourist agency, they used the molecular repurposing to feed the Greeks of 1500 BC. While the Tenth Doctor was trying to stop them, he stole a molecular repurposer and used it to create towers and staircases out of cakes to move. When the Sitheen were beginning to lose, they used the molecular repurposer to escape, turning themselves to sand and reconstituting themselves elsewhere. They tried to go after the Doctor, but he turned their molecular repurposers into sand with his. (PROSE: The Slitheen Excursion)