Welcome to the wonderful world of juvenile pastimes.
A "cow fart" on a window is a specific combination of facial oils and condensation that result from a restricted rapid exhalation on the glass.
The process of laying down the cow fart appears quite clownish to observers from the other side of the glass, particularly as one is first learning the art. There are two parts to the setup of the classic cow fart. The first and most important is the placement of the nose and mouth on the glass. The second is the placement of the hands to restrict the flow of moist air and add body grease. In order to get the essence of the cow fart, your face and hands must be positioned correctly and you must press your face to the glass firmly. As you approach the pane, tilt your head back just a bit so that the bottom face of your nose (where the nostrils exit your face), rather than the front, is squished in to the glass. Once your nose is ‘hooked,’ bring your face back into a more normal level of tilt so that the nose is distended into a pig-like snout flattened into the glass. At this point your hands are more than likely on the glass as a stability measure anyway, so bring them in close to your face so that your thumb and index finger vaguely define a diamond with your face in the bottom center. Your thumbs should be below, but quite near, the corners of your mouth. The impact of your hand placement on the final cow fart is enhanced if you take the time to swab the sides of your nose with the contact edges of your thumbs, to increase the dispersion of facial oil onto the glass.
Once the face and hand placement is accomplished, gently inflate your mouth from your lungs while pressing your lips against the glass. This will widen and change the shape of your lips at the contact seal and begin accreting water vapor on the glass. With as much force as you can muster, suddenly exhale the entire contents of your lungs at top speed and step away from the window. The pattern of condensation and oil will depend on the many variables that go into generating the cow fart. These are as beautiful and unique as snowflakes.
The cow fart will largely fade away in a few seconds, but is also the gift that keeps on giving. The oils that you leave behind will be mostly invisible to casual straight-on observation. But to the close inspector -- particularly from a steep angle, an odd pattern that resists wiping will be evident. After you have so conquered a window, genuine cleanliness can only come from ammonia.
I learned this during my misspent youth (duh!) at a pizza shop in Webster Groves, Missouri. I'm not sure if it has other names in other places, or even if it was unique to my group of friends (maybe we were pioneers!). We were able to bring the cow fart from the Pantera’s Pizza to such illustrious locations as the McDonald's across the street, our high school bathroom mirrors, televisions, and the windows of the local police cruisers.