When poured from a teapot, water has a tendency to run along the underside of the spout. Explain. (See \"The Teapot Effect . . . a Problem,\" by Markus Reiner, Physics Today, September 1956, p. 16.)

The tendency of the water to run along the underside of the spout when it is poured from a teapot is due to the difference in the pressure along the stream of the water and the underside of the spout.
When water flows from the teapot, the speed of the water is lower at the region where it meets with the air. The region where the stream of water touches the lip of the spout has lower speed. According to Bernoulli’s’ equation, the pressure of the stream where it exposed to the air is higher than near the lip of the spout. Thus, the stream of water tends to move towards the underside of the spout due to the pressure difference.