Why was it very difficult for David to think

"David was used to hard work and satisfied with very little food, and he had been as sparing as he could with the bread and water, but after two whole days the bottle was empty and the bread eaten. He could manage without bread but it was dreadful to be so thirsty. In the end he could think of hardly anything but water. " pg. 13

David was so thirsty, he could hardly think.