"Ah, glad you asked! Shells are typically made of calcium carbonate which has at least three different uses." He held up three fingers, he learned his lesson in not including the fourth.

"The first, is that you can mix it with sand and heat it up and make mortar, because that's all mortar is, which is an excellent cement substitute." Senku counted down on his fingers as he explained.

"The second use is agricultural. Using calcium carbonate to neutralize the pH in the soil makes it much more likely that crops would actually grow. If the soil is too acidic, plants don't typically like that and you'll get a horrible yield if you get anything at all. And lastly, it's used to make soap which is vital if you are trying not to get sick."