In those days when animals used to talk in the animal kingdom where the lion was the king. The lizard and tortoise were not good friends, the tortoise envied the lizard and decided to show him this hatred. The tortoise, known for being wise, decided to organize a party. He told other animals to come in the morning and told the lizard to come in the evening when others would have gone to their houses.
On the day of the party, their was plenty of meat and drinks, all animals wined and dined and went home before evening. Tortoise packed all the left over bones and kept them in the kitchen. Meanwhile, the lizard decided not to eat that day so he would eat all at tortoise's house. In the evening, the lizard got to tortoise's house lavished and met everywhere empty. He asked tortoise why everywhere was empty with no animal around. You came a little bit late replied the tortoise, the lizard hungrily asked the tortoise to bring what he could eat, food has finished replied tortoise but I have some leftovers in my kitchen you can eat. Tortoise went in and brought bones for the lizard to eat, due to his hunger he couldn't resist, he ate and had alot stuck to his throat and started nodding to remove the bones.
That's why the lizard nods it's head till date.
NB: this is one of different fables on why the lizard nods it's head.