I replied, "Nice thinking! It's going to get boring staring at the same grey walls after a while."
"Well, what if we look out the windows?" someone asked.
That stopped me for a second. "Windows? There's no windows in a submarine."
"What about the windshield?"
"There's no windshield in a submarine either."
"How do they see where they're driving?"
We had a mini-lesson on SONAR, since many of the students believed that sailors drove submarines around while peering out of the periscope. I'm waiting for when they ask what submarines use for turn signals.