@Buraindo They
can lie, but usually don't because it can bring harm to humans. In this case lying about Yuuna's whereabouts prevented emotional harm to Yuuna, so Ponko's lying was to
prevent harm to a human being. The lie doesn't really harm Yuuna's mother any, and since Gramps was already covering for Yuuna's absence, it won't harm him either. Ponko's lie is perfectly fine under the three laws of robotics.