A man had just settled into his seat next to the window on the plane when another man sits down in the aisle seat and puts his black Labrador in the middle seat next to the man. 
The first man looks very quizzically at the dog and asks why the dog is allowed on the plane? The second man explains that he is a Drug Enforcement Agency officer and the dog is a 'Sniffer dog'.
'His name is Smithy and he's the best there is. I'll show you once we get airborne, when I put him to work.' The plane takes off, and once it has levelled out, the agent says: 'Watch this.'
He tells Smithy to 'search'. Smithy jumps down, walks along the aisle, and finally sits very purposefully next to a woman for several seconds. Smithy then returns to his seat and puts one paw on the agent's arm.
The agent says, 'Good boy', and he turns to the man and says: 'That woman is in possession of marijuana, so I'm making a note of her seat number and the authorities will apprehend her when we land.' 'Say, that's pretty neat,' replies the first man.
Once again, the agent sends Smithy to search the aisles. The Lab sniffs about, sits down beside a man for a few seconds, returns to his seat and this time, he places TWO paws on the agent's arm.
The agent says, 'That man is carrying cocaine, so again, I'm making note of his seat number for the police.' 'I like it!' says his seat mate.
The agent then tells Smithy to 'search' again.
Smithy walks up and down the aisles for a little while, sits down for a moment and then comes racing back to the agent, jumps into the middle seat and proceeds to shit all over the place. 
The first man is really amazed out by this behaviour and can't figure out how or why a well-trained dog would behave like this, so he asks the agent, 'What's going on?' The agent nervously replies,
'He just found a bomb!!!'
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(Courtesy of forwarded mail from Tang Tuck Hoong)