Here's a simple but great joke:
After a month-long holiday in the US , a couple finally boarded the plane in San Francisco last Sunday heading home. As the plane reached cruising speed with the seat belt sign switched off, a 6 ft 3' black man with the build of Mike Tyson in the front row got up from his seat, turned to face the back, raised his arm and yelled, 'HIJACK!'.
Everyone was frozen to the seat, expecting the worst to happen. And two stewards were about to jump onto this guy to overpower him when another voice answered from the back of the plane: 'HI JOHN !'.
The moral of the story is:
If you have a friend named Jack, for heaven's sake don't ever call him in the plane. Otherwise you may land yourself in deep shit.