system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it when you freeze water there’s a ‘foggy’ hard bit in the centre compared to when you boil water first then freeze it and then the ice comes out clear all the way through?
gpt
Regular tap water contains dissolved gasses so when it freezes some of it gets release as small bubbles. So it is the small bubbles that make the ice foggy. 

When you heat up water there are bubbles releases before it boils and that is the same gasses. So if you heat water to boiling and then cool it down you have water with very little dissolved gasses and it freezes to a clear block of ice.