Stiffness

Stiffness is when things don't bend or move easily. Imagine picking up a stick and trying to bend it - if the stick is stiff, it won't bend easily. Now, think about your body. Your muscles and bones work together to move you around. Sometimes, if you sit still for a long time, your muscles and joints can start to feel stiff because they haven't been moving. To make them feel better, you might stretch, jump around or go for a walk to loosen them up.

Now think about other things that can be stiff, like a door that's hard to open because it's stuck. This is because the hinges might be stiff and need to be oiled, or the wood might be swollen from humidity. Or, perhaps you've seen a robot that moves stiffly - that's because the robot's parts are designed to move in a very specific way, so it can't move as smoothly as a human can.

Basically, to be stiff means that something is hard to move or bend, and it can happen with lots of different things, including sticks, our bodies, doors, and even machines.