Answer:
elongate means to make something longer or become longer.
elongate is a verb and elongates is the third person singular form of the verb. Elongates is used with he / she / it or as singular noun subject. For example:
She elongates some words when she is talking. - subject she
The seal's body elongates as it grows. - singular noun subject ( body )
Pencils don't really elongate so a pencil is not a could subject for the verb elongate. Maybe something like this is ok:
The rubber pencil elongates if you pull it.