While his parents were away,,,

[JussstMe]

While his parents were away, a teenaged boy and his friends drank some of his parents' vodka. They then poured water into the vodka bottle to return the level to where it had originally been, and put the bottle back exactly where they had found it. When his parents came home, the father took one look at the bottle of vodka and turned angrily to his son and chastised him for illicit drinking. How did the father know?

[Sphinx_Svens]

his friends were girls and there was lipstick on the bottle?

[xanderma]

It was a new bottle and the seal was now broken?

[xanderma]

or it was a coloured vodka?

[shammydodger]

Before the boy and his friends drank any vodka, the lad marked the level and forgot to rub off the mark afterwards?

[ermintrude35]

...his parents kept the bottle in the freezer...when they added the water it froze....

[imd123]

The Yellow Pages was open under 'French Polishers'?