Muscles become shorter and thicker while contracting.

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Contraction:**
- Muscles are made up of fibers that can change shape. When a muscle contracts, it becomes shorter and thicker. This is a fundamental property of muscles.

2. **Types of Muscle Movements:**
- There are two main types of movements in muscles: contraction and relaxation.
- Contraction occurs when the muscle fibers are pulled together, leading to a decrease in length and an increase in thickness.

3. **Process of Contraction:**
- During contraction, the muscle fibers are activated and they pull on the tendons attached to bones. This pulling action causes the muscle to shorten and thicken, allowing movement to occur.

4. **Process of Relaxation:**
- After contraction, the muscles can relax. During relaxation, the muscle fibers return to their original length and shape, becoming thinner and looser.

5. **Conclusion:**
- The statement "Muscles become shorter and thicker while contracting" is true. This is because contraction leads to a change in the muscle's structure, allowing for movement.