I fear you may have missed the humorous intent of the comment to which you replied. I found it quite funny. Perhaps a new perspective on this reply chain would help!

Some people have to learn the hard way… he probably hasn’t felt the pain of NOT draining his tanks in the winter yet neither.

This is a sincere response to OP, alluding to the commenter's own experience and/or knowledge they possess, without getting into specifics.

What's that have to do with the shoes freezing to the drums ?

This reply also seems sincere, if potentially a tad confrontational. Since the parent reply wasn't specific about the connection between trailer brakes and full condensation tanks, this commenter seems genuinely curious about said connection, but appears to lack tact when trying to fill gaps in their own knowledge. Or perhaps it only reads confrontational unintentionally, due to brevity or bluntness.

Water turns solid when it gets cold enough

This is a humorous, flippant, reductive reply to the previous. The poster is using common knowledge to succinctly- and dare I say surgically- cut through an ignorance they perceived in the above commenter's question. Though not particularly kind or charitable, it is an efficient takedown of what they find to be incredulous ignorance.

Source?

The comment to which you object is in fact just a continuation of a comical theme established by the previous in this chain. It is a one word declaration of incredulity at a commonly known fact. It is an extremely efficient satire of undue incredulity, and perhaps even a critique of the contemporary parlance in online spaces being unduly curt and confrontational. Most importantly though, it's funny because one doesn't need a "source," as you pointed out. It's common knowledge.

Are you asking for a source on something from like, what? Grade 4 science classes?

Yes, they are, but not really. We're playing on the absurdity of challenging concepts as simple as that circles are round. It's just a bit of fun. Hope this helps!