A cat feeling threatened will hiss, growl, and try to attack another animal. While people know glass may keep an invader out, cats and dogs don't necessarily know that. While humans know what they see in a mirror is a reflection of them, animals (like babies) don't necessarily know that.
Cats, like humans, are curious. If there are unexplored areas, they must be explored, whether one is a human or a cat.
Any surface (including human bodies) is for walking on, sitting on, or sleeping on, and anything on that surface is in the way.
(anything that can be moved, should be moved.)
Anything dangling or moving is a toy or prey.
A cat isn't going to politely say, "I'm uncomfortable, please put me down."
Always, always, always pay attention to a cat's body language and eyes. They pretty much always signal when they are in a playful mood or when they are in a not so very playful mood and you are about to be disemboweled. A kitten, unless sleeping (and probably even then), is always in training to become a master hunter.
Wave a piece of food at a cat, and they believe it's a treat for them.