Avraham's Laugh and Ours

In Hebrew he is known as Avram, and later Avraham. In English, he's Abram, and later Abraham.

And in the Bible we see him as an old man laughing. Here is why:

¹⁵ God said to Avraham, “As for Sarai your wife, you are not to call her Sarai [mockery]; her name is to be Sarah [princess]. ¹⁶ I will bless her; moreover, I will give you a son by her. Truly I will bless her: she will be a mother of nations; kings of peoples will come from her.” ¹⁷ At this Avraham fell on his face and laughed — he thought to himself, “Will a child be born to a man a hundred years old? Will Sarah give birth at ninety?”

You've got to sympathize with Avram here. He's got a point. But what kind of laugh is this that he is laughing?

Later in our narrative, his wife Sarah also laughs, and at the same thing. We read there of how ADONAI, Avraham's God, visits Avraham and Sarah. He makes his promise of them bearing a child very specific.  Look at what he says and how Sarah responds:

10 He said, "I will certainly return to you around this time next year, and Sarah your wife will have a son."Sarah heard him from the entrance of the tent, behind him.

11 Avraham and Sarah were old, advanced in years; Sarah was past the age of childbearing. 

12 So Sarah laughed to herself, thinking, "I am old, and so is my lord; am I to have pleasure again?" 

13 ADONAI said to Avraham, "Why did Sarah laugh and ask, 'Am I really going to bear a child when I am so old?' 14 Is anything too hard for ADONAI? At the time set for it, at this season next year, I will return to you; and Sarah will have a son. 15 Sarah denied it, saying, “I didn’t either laugh,” because she was afraid. He said, “Not so — you did laugh.”"