char string1[20], string2[]="Hello world";

    printf("Enter a string:");    // 输入Hello world
    scanf("%s",string1);

    printf("string1 is:%s\n",string1);
    printf("string2 is:%s\n",string2);

输出结果：   
    string1 is Hello
    string2 is Hello world

为什么string2可以输出Hello world，而string1只能输出到Hello呢？？？
Thank you in advance!