liar, liar, pants on fire 

A Chant normally used by children to indicate they think someone is lying.
The full phrase is "Liar, liar, pants on fire. Hangin' on a telephone wire".
It is a paraphrase form an 1810 poem by William Blake titled "The Liar".

Tom: "I didn't brake that lamp. My brother Dick broke it."

Dick: "Liar, Liar, pants on fire!"

liar, liar, pants on fire by procadman January 8, 2012