A very random (I apologise) and belated answer to two plot bunnies at once, sent to me by Spirit of the Skies:
"You were dared to do it?" I demanded.
"Oi couldn' say no," the lad wailed over Mrs. Hudson's cocoa.
"Alfie," I said sternly. "Daring can be a very dangerous habit to indulge in!"
The child sniffled miserably. "Oi didn' know 'twould work!"
"Well you do now, and you're going to be in a lot of pain because of it."
"But oi 'ad to!" the boy wailed, turning pleadingly to Holmes. "Yew understand, don' yew Mr. 'Olmes?"
"Erm…" Holmes stammered. "I…have never licked a lamp-post before, Alfie."
Something in his tone caught my attention; he fidgeted uncomfortably under my scrutiny.
"No, you lived in the country," I said slowly. "No lamp-posts…"
My friend suddenly found his pipe extraordinarily interesting. "It was a pump-handle," he muttered.
Alfie's tears stopped and he stared at the detective for a moment before bursting into a fit of giggles.
Holmes scowled. "Mycroft told me it would only work when the metal got to a certain temperature, so I tested it for a week…one day, it finally worked," he growled.
I could restrain my laughter no longer, chortling with glee at this bit of information.
"'Ow'd yew get loose, Mr. 'Olmes?"
"In a more painful manner than you, young fellow – my brother was considerably less gentle than the Doctor," Holmes mumbled. "He said I screamed like a banshee."