Spit

"To test my theory, do you think you can.... you know, like drool into the spit tube?"

"John, someone will let us out. This is ridiculous!"

"Mom, we are naked and tied up.... or down, or whatever! They could kill us or something! We gotta try this, and spitting in a tube is no big deal"

He watched his mother's expression as she considered and quietly said "fine". The spit tube was the furthest from her and she was clearly straining to reach and put her mouth on it, but she managed. Soon John could see a little wetness flow down the tube... and then nothing happened.

She shifted back, looking at the tube puzzled. "I had hoped something would happen, but that doesn't prove anything. Do we're have to do all four to work, or was that the wrong thing? Or are they JUST MESSING WITH US!"

John considered the options as he tried to calm down his mom. He looked at her crying face on the monitor in front of the tubes... and realized something: he only saw three labels.

"Mom, didn't you say all four tubes have labels? I only see three on the camera."

"One is lower than the others, its probably out of frame. The ... 'cum' one.". Seeing and hearing her discomfort saying that word had a perverse affect on his cock, and he shifted his hips trying to hide the swelling.

"I think I got it... the labels indicate how much you need to fill the tube. The cun one is lower because, well, it would take a long time to produce that much"

He saw the briefest look of devastation cross her face, as she imagined what the future had in store. "That's a lot of spit, and I don't know how we'll fill the other tubes"

"Just try Mom"

She returned to her stretched position, putting the tube in her mouth and holding it there while she summoned spit for it. The tube filled too slowly.

Finally she went back to her normal position. "I need a break, I can't stay like that forever!"

John was feeling impatient, and for more than one reason... it was time to try something else.