You have to descend a 100-meter vertical cliff. However, you have only a 75-meter long rope and a knife with yourself. On the top of the cliff and halfway down - 50 meters above the ground, there are two big pins stuck in it. How can you get safely down to the ground?
First you cut the rope at 25 meters and make a loop at the end of the short part. Then you pass the longer part through the loop, so that you get a 50-meter long rope (the second 25 meters are doubled). You use this rope to descend to the pin and grab it. Then tie one of the ends of the 50-meter rope to the pin, pull the rest back through the loop and let it fall to the ground.