diff --git a/.golangci.yml b/.golangci.yml
index 1f0e4de..94699cc 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -107,7 +107,7 @@ linters-settings:
       # https://github.com/mgechev/revive/blob/master/RULES_DESCRIPTIONS.md#add-constant
       - name: add-constant
         severity: warning
-        disabled: false
+        disabled: true
         arguments:
           - maxLitCount: "3"
             allowStrs: "\"\""
diff --git a/internal/ball.go b/internal/ball.go
new file mode 100644
index 0000000..0b105f2
--- /dev/null
+++ b/internal/ball.go
@@ -0,0 +1,65 @@
+package internal
+
+import (
+	"image"
+	"image/color"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+type ball struct {
+	x      float32
+	speedX float32
+	y      float32
+	speedY float32
+	r      float32
+}
+
+const (
+	ballBaselRadius = 5
+	ballBaseSpeedX  = 1
+	ballBaseSpeedY  = 1
+)
+
+func newBall(screenWidth, screenHeight int) *ball {
+	fScreenWidth := float32(screenWidth)
+	fScreenHeight := float32(screenHeight)
+	r := ballBaselRadius * fScreenHeight / BaseHeight
+	return &ball{
+		x:      fScreenWidth/2.0 - r,
+		speedX: ballBaseSpeedX * fScreenWidth / BaseWidth,
+		y:      fScreenHeight/2.0 - r,
+		speedY: ballBaseSpeedY * fScreenHeight / BaseHeight,
+		r:      r,
+	}
+}
+
+var ballColor = color.White
+
+func (b *ball) draw(img *ebiten.Image) {
+	vector.DrawFilledCircle(img, b.x, b.y, b.r, ballColor, false)
+}
+
+func (b *ball) update(bounds image.Rectangle) error {
+	if b.x+b.r >= float32(bounds.Max.X) || b.x-b.r <= float32(bounds.Min.X) {
+		b.speedX *= -1
+	}
+
+	if b.y+b.r >= float32(bounds.Max.Y) || b.y-b.r <= float32(bounds.Min.Y) {
+		b.speedY *= -1
+	}
+
+	b.x += b.speedX
+	b.y += b.speedY
+
+	return nil
+}
+
+func (b *ball) resize(x, y float32) {
+	b.x *= x
+	b.speedX *= x
+	b.y *= y
+	b.speedY *= y
+	b.r *= y
+}
diff --git a/internal/game.go b/internal/game.go
new file mode 100644
index 0000000..0242250
--- /dev/null
+++ b/internal/game.go
@@ -0,0 +1,89 @@
+package internal
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+)
+
+const (
+	BaseWidth  = 640
+	BaseHeight = 360
+)
+
+type Game struct {
+	width  int
+	height int
+	ball   *ball
+	debug  bool
+	img    *ebiten.Image
+}
+
+var _ ebiten.Game = (*Game)(nil)
+
+func NewGame(width, height int, debug bool) *Game {
+	return &Game{
+		width:  width,
+		height: height,
+		ball:   newBall(width, height),
+		debug:  debug,
+	}
+}
+
+func (g *Game) Update() error {
+	return g.ball.update(image.Rect(0, 0, g.width, g.height))
+}
+
+var backgroundColor = color.Black
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	if g.img == nil {
+		g.img = ebiten.NewImage(g.width, g.height)
+	}
+
+	g.img.Fill(backgroundColor)
+
+	g.ball.draw(g.img)
+
+	if g.debug {
+		g.drawDebug()
+	}
+
+	screen.DrawImage(g.img, nil)
+}
+
+func (g *Game) drawDebug() {
+	ebitenutil.DebugPrint(
+		g.img,
+		fmt.Sprintf(
+			"FPS: %0.2f"+
+				"\nBall: x=%v, y=%v, speedX=%v, speedY=%v, radius=%v",
+			ebiten.ActualFPS(),
+			g.ball.x,
+			g.ball.y,
+			g.ball.speedX,
+			g.ball.speedY,
+			g.ball.r,
+		),
+	)
+}
+
+//nolint:nonamedreturns
+func (g *Game) Layout(outsideWidth, outsideHeight int) (_screenWidth, _screenHeight int) {
+	if g.width != outsideWidth || g.height != outsideHeight {
+		g.ball.resize(float32(outsideWidth)/float32(g.width), float32(outsideHeight)/float32(g.height))
+
+		if g.img != nil {
+			g.img.Dispose()
+			g.img = nil
+		}
+
+		g.width = outsideWidth
+		g.height = outsideHeight
+	}
+
+	return outsideWidth, outsideHeight
+}
diff --git a/main.go b/main.go
index 00880c7..ba5b7ba 100644
--- a/main.go
+++ b/main.go
@@ -1,43 +1,30 @@
 package main
 
 import (
+	"flag"
 	"log"
 
+	"gitea.dwysokinski.me/classic-games/tennis-game/internal"
 	"github.com/hajimehoshi/ebiten/v2"
-	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
-)
-
-type Game struct{}
-
-var _ ebiten.Game = (*Game)(nil)
-
-func (g *Game) Update() error {
-	return nil
-}
-
-func (g *Game) Draw(screen *ebiten.Image) {
-	ebitenutil.DebugPrint(screen, "Hello, World!")
-}
-
-const (
-	screenWidth  = 320
-	screenHeight = 240
-)
-
-//nolint:nonamedreturns
-func (g *Game) Layout(_, _ int) (_screenWidth, _screenHeight int) {
-	return screenWidth, screenHeight
-}
-
-const (
-	windowWidth  = 640
-	windowHeight = 480
 )
 
 func main() {
-	ebiten.SetWindowSize(windowWidth, windowHeight)
-	ebiten.SetWindowTitle("Hello, World!")
-	if err := ebiten.RunGame(&Game{}); err != nil {
+	fullscreen := flag.Bool("fullscreen", true, "")
+	debug := flag.Bool("debug", false, "")
+	flag.Parse()
+
+	ebiten.SetWindowTitle("Tennis game")
+	ebiten.SetWindowResizingMode(ebiten.WindowResizingModeEnabled)
+	ebiten.SetWindowSizeLimits(internal.BaseWidth, internal.BaseHeight, -1, -1)
+	w, h := ebiten.ScreenSizeInFullscreen()
+
+	if *fullscreen {
+		ebiten.SetFullscreen(true)
+	} else {
+		ebiten.SetWindowSize(w, h)
+	}
+
+	if err := ebiten.RunGame(internal.NewGame(w, h, *debug)); err != nil {
 		log.Fatal(err)
 	}
 }