Why do stone walls sweat?

Because the walls cool the air in contact with them, so that its vapor is condensed into water, and deposited on the walls.

Why does the sweating of stones indicate rain?

Because it shows that the air contains a large amount of moisture.

When is a vessel of water said to sweat?

When drops of water collect on the outside of the vessel, it is said to sweat.

Why does a pitcher of cold water sweat?

Because the vapor in the air in contact with the cold pitcher is condensed into moisture, and deposited on the pitcher.

Why does the pitcher sweat most before a rain?

Because the air then contains a large quantity of moisture.