Why among two "nextLine()" only second one is working?

Question

I am using a Scanner class for input String variable but when I use nextLine() for two variable 2nd one is working fine but first one is not working. When I use another Scanner class object then input is working fine. What is the reason behind this?

import java.util.Scanner;

class Demo{

    public static void main (String ... s )
    {
        Scanner scan=new Scanner(System.in);
        int i;
        System.out.println("Enter Number:");
        i=scan.nextInt();

        String name;
        System.out.println("Enter String:");
        name=scan.nextLine();

        String second_name;
        System.out.println("Enter 2nd String:");
        second_name=scan.nextLine();

        System.out.println(i);
        System.out.println(name);
        System.out.println(second_name);
    }   
}