Why does adding another scanf() function prior to one that asks for a string cause issues?

Question

The following code works great:

int main() {
    char inputString[100];
    printf("\nEnter a sentence: ");
    fgets(inputString, sizeof(inputString), stdin);

    printf("\nYou entered: %s\n", inputString);

    return 0;
}

in that it displays the user's sentence on a new line. But as soon as I add in a new scanf() function:

int main() {
        int num;
        char inputString[100];
        printf("Enter a number: ");
        scanf("%d", &num);

        printf("\nEnter a sentence: ");
        fgets(inputString, sizeof(inputString), stdin);
    
        printf("\nYou entered: %s\n", inputString);
    
        return 0;
    }

The program no longer works. Everything set to be printed prints, but it never gives the user an opportunity to enter the sentence. The code just ends after the number is inputted. I don't understand why this is happening, or why this seemingly minor addition causes this error.