Here's why:

* Vinegar is primarily a dilute solution of acetic acid in water. Acetic acid is a polar molecule (meaning it has a positive and negative end), making it easily mixable with water, another polar molecule.

* Like dissolves like. Polar substances dissolve well in other polar substances. Since both vinegar and water are polar, they readily mix.

When you add vinegar to water, the acetic acid molecules disperse evenly throughout the water, creating a homogeneous solution.