It was midnight, and the walls of Professor Zgryza High School looked in the light of the torch lamps like glassy teeth in the jaws of a dark monster. Creeping on tiptoe, Karolina, Kacper and Matthew could barely contain their laughter - they were convinced that a nighttime game of sneaking around the school would be pure entertainment. Referring to the anniversary "I take care of my teeth" campaign, the teachers left clues in the shape of toothbrushes and toothpaste everywhere. As they entered the deserted biology room, their light suddenly fell on something unexpected: a moved piece of parquet floor under the blackboard. Kacper crouched down and began to macerate the boards until one of them gave in under his hand. Instead of a floor, there was a dark crack. They looked at each other, a note of excitement and some fear in their eyes. "I think we need to check it out," Carolina whispered. Matthew, always the first to risk, pulled out his phone and blushed harder. One by one they slipped through the opening, which turned out to be the start of a narrow tunnel. It smelled damp and... something familiar - mint? They descended lower until they finally came to a small cave. The walls were covered with a mosaic depicting a huge jaw - above their heads hung a strangely glittering chandelier in the shape of dental floss. Carol moved closer to the mosaic and saw that buttons in the shape of a toothbrush, floss and toothpaste were hidden between the 'teeth'. Suddenly the lights went out and a low buzzing sound spread through the walls. Kacper reached for his phone, but the screen was dead. Matthew silently cursed. A green light flashed in the corner of the cave and a figure began to emerge from the darkness - tall, slender, with a glittering smile and silver braces. Before they had time to react, a mysterious voice echoed: "Only those who know the true power of oral hygiene will pass on. The rest will stay here... forever." After a moment of silence, it was broken by quiet footsteps and a breath of cool air from the depths of the tunnel. The friends looked at each other, trying to decide whether to run away - or to answer the stranger's riddle....