Mrs. Harris was the first one to break the stunned silence that followed. "Isabel?" she said incredulously, looking at her nursemaid with disbelief.

Isabel did not respond, herself having her steely gaze fixed on Holmes. "Surely you don't believe it was I who murdered the child!" she exclaimed.

"It is precisely what I believe," Holmes assured her cooly.

"On what grounds?"

A small smile formed on Holmes' lips. I knew his full-fledged explanation of deductions was to follow, and it is moments like these that I know the accused is done in, for when one asks for evidence from this man they are sure to receive it. Holmes turned and opened and closed various cabinets in the kitchen. 

"It should be here somewhere. . . Ah!" he exclaimed when he found what he was looking for, "Here. . ." 

He pulled out a dark glass bottle of ethynol and unscrewed the cap, bringing his nose to the rim before placing it on the counter. He then pulled out the kitchen knives, sniffing each one slightly before putting them back, until upon the fifth one he turned around suddenly, brandishing the knife in front of him. 

Mrs. Harris, the one standing closest to him, leapt back in surprise and fright.

Holmes paid her no attention, instead keeping his eyes on Isabel, who was in turn looking at the knife.

"Here is the murder weapon," Holmes told her quietly. "Am I correct?"

Isabel stayed silent, her face unreadable.

"Ah, she needs more evidence," said Holmes, chuckling. He took her hand in his, tracing the back of her hand. He glanced over at me for just a quick second before returning to Isabel, silently calling me over. He took Isabel's other hand, her left hand, and drew it up next to her right. Upon close inspection, trying to see what Sherlock Holmes did, I noticed that the back of her left hand was dry and that the outside of her pinky finger on her left hand had an ink stain, and I suddenly understood. 

Holmes must have seen the change in my face, for he turned to me. "Would you like to do it?" he asked.

I was surprised; explaining all of the connections was usually his to do, but nevertheless I nodded, feeling honored that I had been asked.

"Isabel used the ethynol to wash the knife," I explained.

"All right," sobbed Isabel.

"She was holding it with her left hand," I continued, "her dominant hand, as the ink stain shows, and it dried out her skin." 

"All right," Isabel said again, "yes. Yes, I did it, but I swear, Mr. Holmes, it's not what you think."