In Roald Dahl’s “Lamb to the Slaughter,” Mary implores the police officers to eat the lamb that she used to murder her husband. Mary’s husband was a detective, so she knows that the murder weapon will give her away. The fact that the reader knows that the lamb is the murder weapon, but the police officers do not, is an example of dramatic irony.

The murder in this story is a “perfect crime” because they would be unable to prosecute Mary without the murder weapon. Even though it was not premeditated, Mary still commits the murder and reacts by creating a plan to hide her crime. She notes “how clear her mind became all of a sudden,” and she "began thinking very fast.” She decides that she will “do everything normally.” She wants to keep “things absolutely natural and there’ll be no need for acting at all.”

Mary decides to hide her crime because she is six months pregnant and is worried about the future of the baby. She thinks,

What were the laws about murderers with unborn children? Did they kill them both—mother and child? Did they wait until the baby was born? What did they do?

This uncertainty about her child’s future is Mary’s main motivation for hiding her crime. She even notes that the future punishment “made no difference to her. In fact, it would be a relief.” Therefore, when Mary encourages the police officers to eat the leg of lamb, she is motivated by her unborn child’s future rather than her own.

Mary doesn't force the police officers to eat the leg of lamb she's prepared, but she does encourage it. It smells good, they don't suspect her of the crime, and they are hungry after a long day. The text states,

"Please," she begged. "Personally, I couldn't eat a thing, but it'd be a favor to me if you ate it up. Then you can go on with your work."

The detectives hesitated, but they were hungry, and in the end, they went into the kitchen and helped themselves to supper.

Unbeknownst to the officers, though known to Mary and to us as readers, Mary recently used the frozen leg of lamb they are eating to murder her husband by whacking him on the back of the head with it. They don't suspect her because she is the wife of a fellow officer: to them, she is a social acquaintance rather than a potential murderer. She also looks like a gentle lamb rather than a killer. She is heavily pregnant with a calm demeanor and big, dark eyes.

Mary encourages them to eat the leg of lamb so she can get rid of the murder weapon, which is the piece of evidence that links her to the crime. This is an example of dramatic irony because we as the audience know that the police are eating the evidence while they do not suspect a thing. Mary also seems to get a special enjoyment out of tricking the police this way, as at the end of the story, we learn that she begins to laugh.

Dahl's "Lamb to the Slaughter" is one of the best examples of dramatic irony. At the very end of the story, after Mary has killed her husband with a frozen leg of lamb and the police have come to investigate, Mary ends up feeding the cooked lamb to the policemen, thereby disposing of the murder weapon. While the reader is fully aware of the this, the police continue to enjoy the feast that Mary has provided them.

This act is done to complete the "perfect murder" scenario that Mary has upheld throughout the story. She is the only one who knows that she has a motive to kill her husband; she goes to the grocery store with calm ease; she speaks to the police in a way that communicates both shock and kindness. By ridding herself of the only thing that could point back to her being the murderer, Mary feels relief and almost a giddiness—she has gotten away with murdering her husband. In fact, as the police finish off the leg of lamb, the very last sentence of the short story is, "And in the other room, Mary Maloney began to laugh."

In "Lamb to the Slaughter," Mary forces the police officers to eat the leg of lamb because she does not want her guilt to be revealed to them.

Remember that Mary used the frozen leg of lamb to kill her husband. She hit him over the head with it after he said that he was leaving her. Although her crime was not premeditated, Mary still committed murder, meaning that she would go to prison for a long time if the officers find out what she did.

Instead of facing up to what she has done, Mary concocts a cunning plan. She proceeds to cook the leg of lamb and then offers it to the investigating officers who have stayed late at her house. By doing this, Mary portrays herself as being kind and caring, not a cold-hearted murderess.

This scene, therefore, acts as an example of dramatic irony since we, the reader, know that she is disposing of the murder weapon. The officers, however, think that the meal is just a token of her kindness.