Yarn balls are items in Yoshi's Woolly World and Poochy & Yoshi's Woolly World. They are the equivalent of Yoshi eggs from most Yoshi games. Yarn balls can be thrown at wireframes to create platforms, pipes, windmill blades, Present Boxes or a magic carpet around them and are also used to attack enemies (with some becoming bound by their yarn like Lakitus and Ukikis) and collect items. They can be obtained by swallowing an enemy, hitting a Yarn Basket or via Egg Plants.
Yarn balls come in three different sizes. The smallest size is the most common and breaks apart at only one encounter with an enemy or object, such as a Winged Cloud or a wireframe. Bigger yarn balls can pass through multiple such elements, becoming smaller in the process until they break apart as well. In the battle against Mega Baby Bowser, Yoshi can eat a giant yarn ball that turns into three yarn balls that are approximately his size.
The color of a yarn ball matches the color of the enemy Yoshi swallowed; for example, eating a red Shy Guy results in a red yarn ball. Yarn balls from Yarn Baskets, however, are randomly colored, but might follow a pattern of colors depending on the level's theme and color palette. When a yarn ball is thrown or spit out into the water, it bounces like a skipping stone across it.