Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Prokaryotic cells are extremely small and have a variety of shapes. Most have flagella and a cell wall. They have several other cell structures as well. Their DNA exists as large and small loops. Some prokaryotes form biofilms, which are colonies of cells stuck to a surface. Answer: biofilms
What are colonies of cells stuck to a surface called?