How should I act around my boss who likes to give random long speeches?

My boss is a well respected expert in the field I work in. He’s also surprisingly easy to get along with and is a pretty good guy in general. however, he often gives long speeches randomly in the office and it gets kinda awkward for me because I don’t know what to say; and if I just nod and agree, it looks like I’m not engaging in what he’s talking about and acting like a yes man without any original thoughts. What would be a better way to engage in the topic?

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