When we were raising my sister from 12 years old she used to do this. I don't know if this is the best way and maybe it just worked for us or because she was slightly older but I used to say something like "I love hearing about X because I love how into it you are but I don't really understand it as well as you do so I don't have as much to say. Do you mind if I'm pottering in the kitchen while you tell me?". She didn't care and seemed to love to keep talking at me while I got stuff done and didn't have to sit there 'engaged' in an actual conversation. I would try and make random comments like "I haven't heard of that. How does that work?". She is in her 20s now and still prattles on to me so I guess it didn't shut her up