Lamb to the Slaughter

At the end of "Lamb to Slaughter"by Roald Dahl, Mary Maloney begins giggling because she knows that when the detective said " I bet the evidence is right under our noses" because they were eating the evidence.  She could have also been giggling because her plan work out perfectly. Also, she got away with murder. After the police leave Mary's house I predict she will go up to her room and tell her self how brilliant she is and so much more.