How did the LORD put His words in Jeremiah's mouth at his call?

Answer: He touched his mouth

"The LORD reached out His hand, touched my mouth, and said to me: 'Behold, I have put My words in your mouth.'"

“Then the LORD reached out His hand, touched my mouth, and said to me: “Behold, I have put My words in your mouth.”— Jeremiah 1:9 (BSB)