A common object you’ll see around are small spheres you can sit on. These are known as “Pose Balls”, and are used for easily getting your Avatar into a specific animation. They are often integrated into pieces of furniture such as couches.
They’re often used by romantic couples, so it’s common to see them color-coded blue and pink. Pose balls will generally turn invisible during use.
Some more advanced Pose Balls will give you a menu of various animations to choose from.
As with all Sit objects, use the “Stand” button to unseat yourself from the ball and stop the animation.