There were two grocers, Smith and Jones, in the same street. Smith had a sign in his window, "Avocados, 20 pence a pound". A woman goes in and asks for some. "Sorry love", said Smith, "I haven't got any in just now; come back on Wednesday".

So she goes on up the street to Jones. But his avocados are 2 pounds-fifty a pound! But at least he has them in stock.
"That's a bit steep isn't it? Smith's are only 20 pence a pound".
"Yeah", says Jones, "and when I haven't got any in stock, mine also are only 20 pence a pound!"