Why does we need access modifier "static" for some methods like the following one?

Here is the code I got from the book "Java: The complete reference" by Herbert Schildt on page 239, 240 as PDF. I have researched about "static" but in this case, I wonder why static must be used.

import java.util.Random;

interface SharedConstants {
	int NO = 0;
	int YES = 1;
	int LATER = 3;
	int SOON = 4;
	int NEVER = 5;
}

class Question implements SharedConstants {
	Random rand = new Random();

	int ask() {
		int prob = (int) (100 * rand.nextDouble());
		if(prob < 30) return NO;
		else if(prob < 60) return YES;
		else if(prob < 75) return LATER;
		else if(prob < 98) return SOON;
		else return NEVER;
	}
}

public class AskMe implements SharedConstants {
	static void answer(int result) {
		switch(result) {
			case NO:
				System.out.println("No");
				break;
			case YES:
				System.out.println("Yes");
				break;
			case LATER:
				System.out.println("Later");
				break;
			case SOON:
				System.out.println("Soon");
				break;
			case NEVER:
				System.out.println("Never");
				break;
		}
	}

	public static void main(String[] args) {
		Question q = new Question();
		answer(q.ask());
		answer(q.ask());
		answer(q.ask());
		answer(q.ask());
	}
}

I wonder at the line that created answer method in class AskMe. Why do we need the "static" access control? If not, the compiler will give error like "error: non-static method answer(int) cannot be referenced from a static context".

Thanks in advance.

(This is my first time asking question, if any mistakes, tell me)

You could, but without static you would need an instance of AskMe to call answer in main. Like,

void answer(int result) {
    switch(result) {
        case NO:
            System.out.println("No");
            break;
        case YES:
            System.out.println("Yes");
            break;
        case LATER:
            System.out.println("Later");
            break;
        case SOON:
            System.out.println("Soon");
            break;
        case NEVER:
            System.out.println("Never");
            break;
    }
}

public static void main(String[] args) {
    Question q = new Question();
    AskMe askMe = new AskMe();
    askMe.answer(q.ask());
    askMe.answer(q.ask());
    askMe.answer(q.ask());
    askMe.answer(q.ask());
}