You're encouraged to tell people the truth.
Mindy is asking John at home who ate the last piece of cake that she wanted.
John doesn't want Mindy to be upset that he ate the last piece of cake.
John admits he ate it but tells Mindy he also bought her a brand new one as well.
John tells Mindy he thinks the kids ate it.
Mindy trusts John and is happy to have a whole new cake too.
Mindy gets upset with the kids and later finds the truth and doesn't trust John anymore.