"Since when," a voice echoed out through the warm darkness, "do you have x-ray vision?" Ford Prefect shouted after the dim blur of Arthur Dent, who was beginning to really cramp his style.
"Yesterday," Arthur shouted gleefully. "But I've been able to see in the dark since last week."
"How?" Ford was getting irritated—first these tunnels and then Arthur's newfound superpowers. He could hear the Earthling skipping down the hall ahead.
"I don't know!" Arthur shouted back. "That's the best bit!" Ford was getting sick of all the secondhand elation. "I haven't even been bit by a radioactive spider!"