It is a long, clear bubble that is shaped into a caterpillar, which SpongeBob blows. It has bubble eyes, a mouth, and two antennae.
Role in series
When SpongeBob shows Patrick bubbles from his technique, he uses his wand to create this caterpillar bubble. The caterpillar bubble then starts popping, one segment at a time before it completely disappears. Each bubble makes a different sound when popped.