Each student writes down a question related to the lecture. They pass the question to another student, who has to come up with an answer.
Synchronous online version
Students put questions into a OneNote/Word online collaborative doc. You can create different tabs in the OneNote doc and ask students to work in pairs/small groups.
Asynchronous online version
Each student writes down a question related to the lecture and posts it onto a discussion forum – for others to answer. Alternative use the Q&A forum to ask students a question about the course content, they can only see others contributions once they have posted their own answer.