system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is in hot chocolate powder that makes it effectively unstir-able when it’s at a cold temperature??

Always wanted to know why it can’t be a ‘cold chocolate packet’ too...
gpt
The main reason is that cocoa powder is fatty and fat does not mix well with water. What needs to happen is the water molecules need to get in between the cocoa grains. When water is hot, the water molecules move faster which increases their ability to get in between. When cold, fatty material just sticks together in clumps. Applying shear force (by stirring) helps the water get in between the powder grains. Mixing also works better when it's a small amount of water. The reason for that is the shear force of the spoon is more effective in forcing the molecules to mix. Because it thickens, the clumps can't escape the spoon so easily which also helps. In a full cup, the clumps of fatty material just stick together and just float away from the spoon.