I have always gauged how windy it is outside by the rockin' and rollin' of the water in the toilets inside....never really knew why that happens.  Curiosity got the best of me...after research, Mat  Soniak, on Mental Floss.com has our answer.

He says, it has something to do with the "vent stack" and the air pressure,  part of Bernoulli's principle.

When the velocity of wind is high, it makes a suction effect in the plumbing system that pulls on the water in your commode, thus creating the movement.  Click on the Mental Floss link above for Mat's full explanation.  Thanks Mat, mystery solved!

Do you have a "why" question that I can help answer?  Would LOVE to tackle it....comment below and let's get to the bottom of it!!!