You're guaranteed a good photo when a cat yawns. It's so cute because it looks like they're roaring. It's really simple to capture, you just have to wake them up with your camera/phone ready to take photos.
Here's the best one I've captured yet. The cat tree that Dorian was on is right next to a window in our sunroom so it's a great location for clear photos with a white background.
Mona's yawn is harder to capture because she sleeps in closets and/or rooms we aren't in (so the lights aren't on.) But, here's some of her yawns I've captured years ago.
(And I'm sure you can tell by these photos that I've gotten better with photography over the years. These aren't even completely focused. But still, I'm glad to have them--Mona was 6 months old here.)