A row of Thorns.
Thorns, or Crystals are spiky obstacles found in Super Mario World 2: Yoshi's Island and its sequel Yoshi's Island DS, they are found mostly in castles. Contact with them will immediately kill a Yoshi, making them very dangerous. Luckily, a Yoshi Egg will destroy a single thorn, though since thorns are usually found in groups and Yoshi can only carry six Yoshi Eggs at a time, most thorn groups are impossible to completely destroy. However, in Yoshi's Island DS Baby Donkey Kong explosive eggs can easily destroy a group of them in one shot. Sometimes after destroyed the thorns reveals Winged Cloud and in The Potted Ghost's Castle one specific thorn even reveals a door.