Magic Finger is a simple experiment I conducted with my little brother, David. It was really cool and he and I both enjoyed it. All you need is black pepper, soap, and a large-sized pop bottle cut in half. You can use either the bottom half or the top part. We used the top half since we didn’t need a lot of water.
Fill the bottle half with water and sprinkle some black pepper into it. My brother placed his finger in the mixture. Of course nothing happened. Then I asked him to rub his finger against the soap until it was wet.
Then David placed his finger back inside the mixture of water and pepper. What happened next was amazing. The pepper was repelled away from his finger and scattered to the sides of the bottle. He and I were both stunned.
It turns out that the soap molecules relaxed the water’s surface, causing it to push outward. The soap actually changed the surface tension of the water. This was a little difficult to explain to my brother, who is only eight. So, I told him that water molecules separated, forcing the pepper to go with them.