Sound vibrations travel through liquids, gases (like air), and solids (like the string in this activity). Sliding your fingers along the string creates friction (rubbing and sticking). This causes the string to vibrate. The vibrations travel up the string to the cup, which acts like a speaker and amplifies them (makes them sound louder). But why does the wet string work better than the dry string? The wet string made your fingers stick and rub more, causing more vibrations and more sound. In this "Fetch!" activity, turn an ordinary cup and string into a screeching, squawking sound machine.