In England nobody under the age of eighteen is allowed to drink in a public bar． Mr． Thompson used to go to a bar near his house quite often，but he never took his son，Tom，because he was too young． Then when Tom had his eighteenth birthday， Mr．Thompson took him to his usual bar for the first time．They drank for half an hour， and then Mr．Thompson said to his son，“Now， Tom，I want to teach you a useful lesson．You must always be careful not to drink too much． And how do you know when you've had enough？ Well， I'll tell you．Do you see those two lights at the end of the bar？ When they seem to have become four，you've had enough and should go home．” “But， Dad，” said Tom，“I can only see one light at the end of the bar．”