The second ghost

Ghost2…has a story for you…

It was a cold Sunday afternoon in December. Snowflakes were swirling past the living-room windows of 221B Baker Street. The tea which Mrs. Hudson had prepared one and a half hours earlier was slowly dwindling while the three present warmed themselves at the roaring fire.
»You know«, Mike Stamford was saying while he fished the last cucumber sandwich of Mrs. Hudson’s delicate étagère, »the new apprentice who’s been in my department for about four weeks now, is kind of funny… Oh, thanks, John!«
John had poured him another cup of tea and was now throwing a fifth piece of Mrs. Hudson’s self-baked shortbread over to Sherlock who was sitting cross-legged on the sofa.
»Why’s your apprentice funny?«, he asked then.
»Well,« Mike proceeded after taking a sip of tea, »on Wednesday he’d returned a book to me which I’d borrowed him for further studies of the basics. On Friday just when we were about to call it a day he suddenly came and told me he had forgotten £20 in the book. He even still knew between which pages he’d used it as a bookmark. Pages 79 and 80, he said. Well, as I’d already put the book back on its shelf in the archive I offered him to take a look on Monday. But he said he needed the money back at once because he wanted to take his girl-friend to the pictures on the weekend.«
»To use £20 as a bookmark is indeed rather unusual,« John said.
»Though it’s still better than biscuits,« he added, shooting a glance in the sofa’s direction but Sherlock ignored him and continued to crumble his shortbread unaffectedly.
Mike hid a smile and continued, »the boy asked if I could give him the £20 and fetch the note between the pages of the basics‘ book on Monday. I guess if I had had the money I would have given it to him but I only had some loose change on me. When I told him this he became angry and stormed off.«
Sherlock got up from the sofa and to John’s annoyance, shook off shortbread crumbles onto the floor. Then he sat down again and said:
»It was a good thing you didn’t have £20, Mike. The boy lied to you.«

How does Sherlock know?

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