Maybe a better question is: “What did He write in the sand?” John is the only author who records this seemingly bizarre set of events for us. In chapter 8:1-11 of his epistle we read of a woman who were caught in the act of adultery and brought to Jesus by the pharisees. After Jesus heard her story being told by the teachers of the law, He bends forward and writes something in the sand with His finger. But why and what is it that He writes? Jesus here uses a familiar rabbinic method of taking a practical gesture to teach a spiritual lesson. The gesture would be done in such a way that the learner(s) would have to arrive at the answer themselves.
Now some offer a number of explanations but I believe the key to solving this puzzle lies written in the previous chapter… to a feast the Jews celebrated every year. John writes in chapter 7:37: “Now on the last and most important day of the feast, Jesus stood and called out [in a loud voice], “If anyone is thirsty, let him come to Me and drink!” Part of the feast of Tabernacles was a ritual performed by the High Priest where he would take a cup, brought to him by someone who have taken water from the pool of Siloam (which means ‘sent One’ in Hebrew) and pour it on the altar and then quotes the verse from Jeremiah 17:13: “O Lord, the Hope of Israel, all who forsake You shall be put to shame. They who depart from You shall [disappear like] writing upon the ground, because they have forsaken the Lord, the Fountain of living waters.“
When Jesus bent down to write on the ground for a second time, the scribes & pharisees realized that He was speaking about them and started to depart eldest to the youngest. They knew that none of them were innocent enough to cast the first stone.