Simple bouncy balls anim

/* Note to pastebin viewers */
/* Create a PictureBox on your form and set Dock property to full */
/* Rename created PictureBox to pb */

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace SimpleAnimation
{
    public partial class Form1 : Form
    {
        /// <summary>
        /// Store information about balls. Positions, speeds etc
        /// </summary>
        List<Ball> balls = new List<Ball>();

        /// <summary>
        /// Set event handlers
        /// </summary>
        public Form1()
        {
            InitializeComponent();

            pb.MouseDown += new MouseEventHandler(pb_MouseDown);
            pb.Paint += new PaintEventHandler(pb_Paint);
        }

        /// <summary>
        /// Updates and draw balls on screen
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void pb_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.DrawString("Left click to create a ball", SystemFonts.DefaultFont, Brushes.Black, new PointF(15, 15));
            e.Graphics.DrawString("Right click to remove last ball", SystemFonts.DefaultFont, Brushes.Black, new PointF(15, 30));

            for (int i = 0; i < balls.Count; i++)
            {
                Ball b = balls[i];

                //Screen bounds
                int top = 0;
                int right = pb.Width - b.Size;
                int bottom = pb.Height - b.Size;
                int left = 0;

                //Left wall
                if (b.PosX < left)
                {
                    b.PosX = left;
                    b.VelX *= -1;
                }

                //Right wall
                if (b.PosX > right)
                {
                    b.PosX = right;
                    b.VelX *= -1;
                }

                //Top wall
                if (b.PosY < top)
                {
                    b.PosY = top;
                    b.VelY *= -1;
                }

                //Bottom wall
                if (b.PosY > bottom)
                {
                    b.PosY = bottom;
                    b.VelY *= -1 + 0.5f; // Add a bounce
                }

                //Gravity
                if (b.PosY < bottom)
                {
                    b.VelY += 0.25f;
                }

                //Update positions
                b.PosX += b.VelX;
                b.PosY += b.VelY;

                //Draw ball
                e.Graphics.FillEllipse(Brushes.Black, b.PosX, b.PosY, b.Size, b.Size);

                balls[i] = b;
            }
        }

        /// <summary>
        /// Left click - Create ball
        /// Right click - Remove last ball
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void pb_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == System.Windows.Forms.MouseButtons.Left)
            {
                balls.Add(new Ball(e.X-(Ball.DEFAULT_SIZE/2), e.Y-(Ball.DEFAULT_SIZE/2), 0, 1));
            }
            if (e.Button == System.Windows.Forms.MouseButtons.Right)
            {
                if (balls.Count > 0)
                {
                    balls.RemoveAt(balls.Count - 1);
                }
            }
        }

        /// <summary>
        /// Setup game timer to run every 10 ms
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Form1_Load(object sender, EventArgs e)
        {
            Timer gameTimer = new Timer();
            gameTimer.Tick += new EventHandler(gameTimer_Tick);
            gameTimer.Interval = 10;
            gameTimer.Enabled = true;
        }

        /// <summary>
        /// Game timer tick, redraw picture box
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void gameTimer_Tick(object sender, EventArgs e)
        {
            pb.Invalidate();
        }
    }

    public class Ball
    {
        public float PosX { get; set; }
        public float PosY { get; set; }
        public float VelX { get; set; }
        public float VelY { get; set; }
        public int Size { get; set; }
        public const int DEFAULT_SIZE = 25;

        public Ball(int x, int y, int speedx, int speedy, int size = DEFAULT_SIZE)
        {
            PosX = x;
            PosY = y;
            VelX = speedx;
            VelY = speedy;
            Size = size;
        }
    }
}