Why do you use a glass rod when pouring liquid?

Glass rods are used when pouring liquids in order to avoid spills. When a glass rod is laid against the pouring edge of a beaker, it causes the liquid inside to flow along the rod and down into the receiving vessel instead of splashing over the lip.

The surface tension of water can cause it to behave erratically when poured. In some cases, the liquid may "stick" to the pouring vessel, and instead of flowing off in a clean stream, it may flow down the side of the beaker and splatter. Using a glass rod or other object reduces this potential by allowing the water to stick to the rod instead.