Given the task definition and input, reply with output. In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.

Louisa Glasson: Oh , and read him a bed time story . He seems to like the one about the talking fire engine .
Dr. Martin Ellingham: Hm . Highly unlikely he can grasp the concept of a fire engine , let alone _ one _ that 's apparently sentient .
engine