why can't I create a method in the main method

why can't I create a method in the main method

Hello I am a want create method into main? So this is code where i want
crete method:
import java.io.*;
import java.util.Random;
class input{
public static void main (String args[]){
void Randomises() {
int writabledata;
Random a=new Random();
writabledata=a.nextInt();
}
}}