MINNEAPOLIS â During their planetarium field trip, the third-graders from Castle Elementary School in Oakdale, Minn., had plenty on their minds. Is there any water on the moon? Which is the coldest planet, and the hottest one? What is a black hole? Would you freeze to death on Mars? Kaitlin Ehret, the Bell Museum planetarium educator leading the field trip, smiled as the students peppered her with more questions â in the âchatâ window of a Zoom call. One by one, she called on them in the way theyâve become accustomed to this year: by telling them she would un-mute them so they could share their thoughts with the class.