Genesis 18:13 - 15

"Then the Lord said to Abraham, 'Why did Sarah laugh and say "Will I really have a child, now that I am old?" Is anything too hard for the Lord? I will return to you at the appointed time next year and Sarah will have a son.' Sarah was afraid, so she lied and said, 'I did not laugh.' But he said, 'Yes, you did laugh.'" NIV translation

We see from this conversation that the Lord knows what is in our hearts and our minds. Sarah had said those things to herself and yet He knew what she said. When asked about it, she lied and that reminds us of one of the ways that the devil tries to separate us from God. The Lord had asked a question not in condemnation but to strengthen their faith but the devil brought fear into her heart and she lied. 

When we have trouble seeing past the physical, we can admit it to God and He will still accept us. When we try to hide it, we open a door to the tricks of Satan.

Genesis 18:16 - 19

"When the men got up to leave, they looked down toward Sodom, and Abraham walked along with them to see them on their way. Then the Lord said, 'Shall I hide from Abraham what I am about to do?' Abraham will surely become a great and powerful nation, and all nations on earth will be blessed through him. For I have chosen him, so that he will direct his children and his household after him to keep the way of the Lord by doing what is right and just, so that the Lord will bring about for Abraham what he has promised him.'" NIV translation

As they left, Abraham walked with them and the Lord began to reveal their purpose. We see a contrast between the blessing planned for Abraham and the judgment that awaited Sodom and Gomorrah.