Mystery Crate
Mystery Crates are objects in Yoshi's Story that Yoshi can Ground Pound to destroy. Melons are usually inside these boxes, though in Surprise!!, an Air Bag can pop out of one.
Other uses
Your Yoshi can also push these crates around an area so it's easier to get from one place to another. They can also do it to earn prizes. When multiple Mystery Crates are pushed together, They'll be awarded melons. The amount of melons depends on how many are pushed together.
Sometimes, these are used to get onto higher platforms, though rare.