What is the external stimulus in thigmotropism?

Thigmotropism is defined as a negative or positive directional response in a plant or part of a plant such as a plant organ, that is caused by an external stimulus such as touch or contact with any solid object. The factor that causes the stimulus is a hard surface that can change the direction of growth of the plant or part of the plant.  Hope this helps :)

Explanation:

i'd say that the moon rock is about 2.2 billion years old.