  Conversational teachable agents offer a promising platform to support
learning, both in the classroom and in remote settings. In this context, the
agent takes the role of the novice, while the student takes on the role of
teacher. This framing is significant for its ability to elicit the Prot\'eg\'e
effect in the student-teacher, a pedagogical phenomenon known to increase
engagement in the teaching task, and also improve cognitive outcomes. In prior
work, teachable agents often take a passive role in the learning interaction,
and there are few studies in which the agent and student engage in natural
language dialogue during the teaching task. This work investigates the effect
of teaching modality when interacting with a virtual agent, via the web-based
teaching platform, the Curiosity Notebook. A method of teaching the agent by
selecting sentences from source material is compared to a method paraphrasing
the source material and typing text input to teach. A user study has been
conducted to measure the effect teaching modality on the learning outcomes and
engagement of the participants. The results indicate that teaching via
paraphrasing and text input has a positive effect on learning outcomes for the
material covered, and also on aspects of affective engagement. Furthermore,
increased paraphrasing effort, as measured by the similarity between the source
material and the material the teacher conveyed to the robot, improves learning
outcomes for participants.
