When the Sun rises, the hills untie their bonnets. This metaphorical expression refers to the visual phenomenon that occurs when the Sun appears on the horizon and the hills no longer appear dark and shadowy.

Explanation:
- The Sun rises every morning, marking the beginning of a new day. As the Earth rotates, different parts of the planet are exposed to the Sun's light, causing the appearance of sunrise in various locations.
- The phrase "the hills untie their bonnets" is a poetic way of describing how the hills, which were previously shrouded in darkness, become illuminated by the Sun's rays. It suggests that as the Sun rises, the hills remove their coverings or "bonnets" and reveal their true beauty.
- This expression is figurative and highlights the transformative power of the Sun's light. It implies that the landscape undergoes a visual change, becoming more vibrant and colorful as the Sun rises.
- The golden hues of the rising Sun cast a warm glow on the hills, enhancing their contours and bringing out their natural features. Shadows recede, and the landscape becomes more defined and picturesque.
- The phrase also evokes a sense of anticipation and renewal. The Sun's rising symbolizes a fresh start, a new day filled with opportunities and possibilities. It signifies the beginning of a cycle, where darkness gives way to light, and the world awakens from its slumber.
- Overall, the expression "the hills untie their bonnets" captures the beauty and transformative nature of a sunrise, as the Sun's light illuminates and enhances the landscape, bringing about a sense of renewal and new beginnings.