Task: Question Answering
Input (English): Context: Howard pulled out his wallet from his trousers and threw a fiver onto the bar , but the barman shook his head and mouthed ' Tenner ' . Howard was silently outraged at the price of a whiskey but consoled himself with the thought that he did n't go out often , so he may as well spend up big . The whiskey sour was sharp on his throat and he almost gagged at the smell , but after the second swallow , it was n't so bad . Howard saw a shadow sliding up towards him . 
Question: What is the reason the barman shook his head and mouthed tenner ?
Output (English): Howard did n't put enough money on the bar to pay for his tab .