Much of the world knows the Grenadier Guards from their roles as formal guards in London and at Windsor Castle. Their distinctive ceremonial uniforms are a symbol of the British Crown. They are also one of the world's best light infantry units who joined the British Army in the recent wars in Iraq and Afghanistan. But while they left their distinctive bearskin hats at home, they did bring many traditions to desert wars with them.
"Make way for the Queen's Guard."
One of those traditions surrounds saying the word "yes." Apparently, members of the Grenadier Guards find the affirmative to be redundant. According to one guardsman in a 1989 BBC story called the "Weird and Wonderful Traditions of the Grenadier Guards," saying yes is redundant as a guardsman always obeys his orders. The only alternative would be saying "no," something a guardsman would never do.
So in the Grenadier Guards, they simply respond to direct orders with "sir."
The Grenadiers' unquestioned obedience doesn't limit their ability to communicate. According to a ranking Guardsman of the time, they can still pass different meanings through the word using different tones and inflections. If you're given a bad assignment, you just say "sir." It doesn't mean you're happy about it, but you accept it. If you're told something you simply just can't believe, you might say "sirrrrrr?" Or maybe you disagree with it entirely and don't like it one bit.
A monotone "sir" is an acceptable response. Tune in to the above video at around 6:15 to hear the Guards tell it.