Weight ball. It's a weighted ball. I bought it after being recommended by a teacher at a cram school for elementary school entrance exams. They said that after throwing this, you'll be able to throw a regular ball quite far. They also said not to overdo it, as it could hurt your shoulder, so to limit it to a few times a day. I thought I'd lost it and ordered one on Amazon, but then I found it, so I haven't used it at all. Thank you.