"It became a necessity." Senku chuckled. "I suppose it is. I can show you how to make it. A knife doesn't even take very long once you know what you're doing."

He'd done all the trial and error work ages ago.

"I had nothing but theories and knowledge from the past when I had to figure out how to make my own stone tools. It's a good trick to have though." It meant you could dump him somewhere with nothing and he'd probably survive.