Error on randomizing strings in a list C#

Coolai

I want to create a program that shows a random category (from the list of category I've created) with the right words in a message box whenever I click the button.
[img=http://i717.photobucket.com/albums/ww176/T3ZTAM3NT/1stSSonError_zpsb1007b5d.jpg]
The categories is randomized when I run it but the right word that should be with the category isn't correctly placed.
[img=http://i717.photobucket.com/albums/ww176/T3ZTAM3NT/2ndSSonError_zps1a838970.jpg]
Also, I know that the program will crash once the program reaches a negative index of a category or once all of the category is shown.

Code:
[code]namespace randomCategory
{
public partial class Form1 : Form
{

Random rand = new Random();
List categories = new List { "Book Titles", "Movie Titles", "Car Parts", "Human Body Parts", "Transportations" };


public Form1()
{
InitializeComponent();
listBox1.DataSource = categories;
}

public void selection()
{
// logic for setting a random category
int index = rand.Next(categories.Count);
var category = categories[index];


// logic for assigning the word for a category
switch (index)
{
case 0:
MessageBox.Show(category, "Harry Potter");
break;
case 1:
MessageBox.Show(category, "Summer Wars");
break;
case 2:
MessageBox.Show(category, "Bumper");
break;
case 3:
MessageBox.Show(category, "Eyes");
break;
case 4:
MessageBox.Show(category, "Boat");
break;
default:
MessageBox.Show("Empty!", "!!!");
break;
}

categories.RemoveAt(index);
}

private void button1_Click(object sender, EventArgs e)
{
selection();
}

private void Form1_Load(object sender, EventArgs e)
{

}
}
}
[/code]