Refueling Puns

A large Russian company buys a batch of off-road Mercedes trucks. The deal also includes training of drivers to work with the new equipment. One such Mercedes catches up with an old MAZ truck on the road. At one point the MAZ stopped, then a man with a bucket jumped out of it and scooped some water from a puddle on the road and poured it into the fuel tank. Then he gets in the truck, lights it and sets off. The instructor in the Mercedes, who speaks a little Russiаn, asks the driver:

- What's happening?

"It's out of fuel."

"You keep fuel in puddles along the road?"

- No. This is water.

The German was silent for a few minutes and stated:

- The car in front of us is moving with water!

- No! It's moving with diesel!

The instructor decides to make fun of him and is silent until they reach the base. Excited, he goes there and shares with his colleagues what he saw. It turns out that someone else saw the same thing, but he doesn't know Russiаn and couldn't ask. He returns to his trainee and continues to question. If MAZ is running on diesel, why did the driver add water to the fuel tank? The Russiаn explains to him that the pipe that sucks the diesel is located just a few centimetres above the bottom of the fuel tank. At the moment it stops refuelling, there are another ten litters of diesel in the fuel tank. When water is poured, the level rises, then the diesel, which is lighter, rises from the top and the truck can travel many more kilometres. Amazed by this explanation, the German asks:

"Why don't they put the fuel pipe at the bottom of the tank?"

The Russiаn's in shock answers him:

- But what if there is water in the diesel?

Edit: How a MAZ truck looks like - https://youtu.be/roj5Xf55PDU