They were using this question as a trap, in order to have a basis for accusing him. But Jesus bent down and started to write on the ground with his finger. When they kept on questioning him, he straightened up and said to them, “Let any one of you who is without sin be the first to throw a stone at her.”
What was Jesus writing on the ground? My first suggestion was that this was a nervous tick. Jesus, as a celibate Jew was embarrassed by the situation. A long standing interpretation of the church is that he wrote a part of Jeremiah 17:13: Those who turn away from you will be written in the dust because they have forsaken the LORD, . Alternatively he may have written from Exodus 23: 1: Do not help a guilty person by being a malicious witness.
I like the idea, though that he was writing down the names of the accusers and listing some specific sins that they were guilty of.
In any case the wonderful statement "Let any one of you who is without sin be the first to throw a stone at her." is a tremendous example of Christian tolerance.