Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: A physical change in matter is a change in one or more of matters physical properties. Glass breaking is just one example of a physical change. Some other examples are shown in Figure 3.16 and in the video below. In each example, matter may look different after the change occurs, but its still the same substance with the same chemical properties. For example, smaller pieces of wood have the ability to burn just as larger logs do. MEDIA Click image to the left or use the URL below. URL: Because the type of matter remains the same with physical changes, the changes are often easy to undo. For example, braided hair can be unbraided again. Melted chocolate can be put in a fridge to re-harden. Dissolving salt in water is also a physical change. How do you think you could undo it? Did you ever make a "volcano," like the one in Figure 3.17, using baking soda and vinegar? What happens when the two substances combine? They produce an eruption of foamy bubbles. This happens because of a chemical change. A chemical change occurs when matter changes chemically into an entirely different substance with different chemical properties. When vinegar and baking soda combine, they form carbon dioxide, a gas that causes the bubbles. Its the same gas that gives soft drinks their fizz. Not all chemical changes are as dramatic as this "volcano." Some are slower and less obvious. Figure 3.18 and the video below show other examples of chemical changes. MEDIA Click image to the left or use the URL below. URL: How can you tell whether a chemical change has occurred? Often, there are clues. Several are demonstrated in Figures 3.17 and 3.18 and in the video below. MEDIA Click image to the left or use the URL below. URL: To decide whether a chemical change has occurred, look for these signs: Gas bubbles are released. (Example: Baking soda and vinegar mix and produce bubbles.) Something changes color. (Example: Leaves turn from green to other colors.) An odor is produced. (Example: Logs burn and smell smoky.) A solid comes out of a solution. (Example: Eggs cook and a white solid comes out of the clear liquid part of the egg.) Because chemical changes produce new substances, they often cannot be undone. For example, you cant change a fried egg back to a raw egg. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo the tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar reacts with the tarnish. This is a chemical change that makes the pennies bright and shiny again. You can try this yourself at home to see how well it works. If you build a campfire, like the one in Figure 3.19, you start with a large stack of sticks and logs. As the fire burns, the stack slowly shrinks. By the end of the evening, all thats left is a small pile of ashes. What happened to the matter that you started with? Was it destroyed by the flames? It may seem that way, but in fact, the same amount of matter still exists. The wood changed not only to ashes but also to carbon dioxide, water vapor, and other gases. The gases floated off into the air, leaving behind just the ashes. Assume you had measured the mass of the wood before you burned it. Assume you had also trapped the gases released by the burning wood and measured their mass and the mass of the ashes. What would you find? The ashes and gases combined have the same mass as the wood you started with. This example illustrates the law of conservation of mass. The law states that matter cannot be created or destroyed. Even when matter goes through physical or chemical changes, the total mass of matter always remains the same. (In the chapter Nuclear Chemistry, you will learn about nuclear reactions, in which mass is converted into energy. But other than that, the law of conservation of mass holds 
 Question: After a physical change, matter may
look different.

Context: A joint is a point at which two or more bones meet. There are three main types of joints in the body: 1. Fixed joints do not allow any bone movement. Many of the joints in your skull are fixed ( Figure 1.1). There are eight bones that fuse together to form the cranium. The joints between these bones do not allow movement, which helps protect the brain. 2. Partly movable joints allow only a little movement. Your backbone has partly movable joints between the vertebrae ( Figure 1.2). The skull has fixed joints. Fixed joints do not allow any movement of the bones, which protects the brain from injury. 3. Movable joints allow the most movement. Movable joints are also the most common type of joint in your body. Your fingers, toes, hips, elbows, and knees all provide examples of movable joints. The surfaces of bones at movable joints are covered with a smooth layer of cartilage. The cartilage reduces friction between the bones. Ligaments often cross a joint, holding two nones together. For example, there are numerous ligaments connecting the leg bones across the knee joint. Four types of movable joints are discussed here. 1. In a ball-and-socket joint, the ball-shaped surface of one bone fits into the cup-like shape of another. Exam- ples of a ball-and-socket joint include the hip ( Figure 1.3) and the shoulder. 2. In a hinge joint, the ends of the bones are shaped in a way that allows motion in two directions, forward and backward. Examples of hinge joints are the knees ( Figure 1.4) and elbows. 3. The pivot joint ( Figure 1.5) only allows rotating movement. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of your hand up and down. 4. A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over the other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small side-to-side motions. There are also gliding joints in your ankles. 
 Question: what type of joint allows you to turn the palm of your hand?
a pivot joint

Context: Mondas is a fictional planet in the British science fiction television series Doctor Who. 
 Question: In what fictional work would you find a character named Mondas?

doctor who