Grammaticalization is the process by which a lexical item (e.g., noun) acquires a more functional role (e.g., preposition) over time. Grammaticalization is considered largely unidirectional, that is, change from functional to lexical is far less common (Hopper & Traugott, 2003). What is the cause of this unidirectionality? Our experiment tests whether individuals have a preference in the direction of grammaticalization when performing semantic extension in communication. We focus on the phenomenon of using body part nouns as a source of spatial prepositions. We predicted that participants extending body parts to use as prepositions would find the task easier and more intuitive than participants extending prepositions to use as body parts. However, our results show no directional bias, indicating that the historical unidirectional tendency for body parts to be used as spatial relations does not originate in a bias that individuals have for using one to refer to the other.