You must have noticed that every time you pour some juice or water in an ice filled glass, something makes a cracking sound.. Are you so clumsy that every time you do that you break the glass or is it something else that makes the cracking sound?? Of course it’s something else! It’s the ICE! But why does it happen? Hmm for that you will have to first understand how ice is made.
The water is a liquid that fits itself into the shape of any container you pour it in. Though water looks clear but it is made up of a large number of tiny particles that keep moving. Now these particles constantly move around when water is in a liquid state. When water is heated up these particles start to move faster. In fact, they move so fast that when water is boiling, you can see the restlessness in form of bubbles at the top of the boiling water. On the other hand, as the temperature of water gets lower, that is when water gets cold, these particles become very slow. Eventually, when water is very cold the particles hardly move and water turns into ice!!
Now when we put water over the ice, the outside layer of ice starts to warm up immediately. This means particles on the outside of ice start moving faster. The ice on the inside remains freezing cold and therefore the particles move very little. This builds up uneven stress on the cube of ice and it cracks up.
Take a look at this video to understand even better: