You've no doubt seen your cat sleep and make body movements and noises as if they were dreaming. I have two cats that regularly talk in their sleep, mostly humming sounds, and I once caught another one chattering in his sleep. Will we ever know what they are dreaming about? You might be amazed at how well science has figured that out. By recording brain waves during the activities an animal does while awake and comparing them to brain waves recorded during sleep, we can match them up and see what's going on in our critters' dreams. Rats replay the new tasks they've learned that day. Birds sing their songs in their heads while they sleep. Cephalopods practice threat response while they sleep, which sounds like a nightmare. Cats also replay their daytime activities in their sleep, as Savannah Geary of SciShow explains. This video is 6:45; the rest is an ad.