Why do ice cubes crack when dropped in a drink?

Freezer ice is often well below 0°C, around -18°C in a typical home freezer. When it hits a much warmer liquid, the surface layer heats rapidly while the centre stays very cold. Ice, like most solids, changes size with temperature, so the outside and inside try to expand by different amounts. This uneven change, called thermal stress, creates internal forces strong enough to crack the brittle ice, producing the popping and crackling sound. Trapped air bubbles and impurities act as weak points, which is why cloudy ice tends to crack more readily than clear ice.