What motion is being used when you stand on your tiptoes to grab something off of the top shelf?

Choice A reason: Symphyses are cartilaginous joints where bones are joined by fibrocartilage. They allow limited movement, such as in the pubic symphysis, but are not the most movable joints.

Choice B reason: Synovial joints are the most movable type of joints in the human body. They feature a joint cavity filled with synovial fluid, articular cartilage, and a fibrous capsule, allowing for a wide range of motion. Examples include the shoulder, hip, and knee joints.

Choice C reason: Syndesmoses are fibrous joints where bones are connected by ligaments or interosseous membranes. They allow more movement than sutures but are still relatively limited compared to synovial joints.

Choice D reason: Gomphoses are immovable joints found between teeth and their sockets. These joints are highly stable but do not permit movement.

Choice A reason: The buccinator muscle compresses the cheek, aiding in actions like blowing and chewing. While it contributes to facial expression, it is not the primary muscle responsible for smiling.

Choice B reason: The zygomaticus muscle is the correct answer. It originates from the zygomatic bone and inserts into the corner of the mouth. Its contraction pulls the mouth upward and outward, producing a smile. It is the principal muscle involved in smiling.

Choice C reason: The mentalis muscle elevates and wrinkles the skin of the chin and protrudes the lower lip. It is involved in expressions of doubt or displeasure, not smiling.

Choice D reason: The orbicularis oris encircles the mouth and controls movements of the lips, such as puckering or closing. It is essential for speech and kissing but not for smiling.

Choice E reason: The masseter is a major muscle of mastication (chewing). It elevates the mandible to close the jaw and has no role in facial expressions like smiling.