diff --git a/README.md b/README.md
index d2af656..fe6c552 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,12 @@
 # Minesweeper using Rust & WASM
 
 Tutorial: https://www.youtube.com/watch?v=0ywizYLPV00
+
+## Development
+
+```
+wasm-pack build --target web
+serve
+```
+
+If it says that you don't have `serve`, you can install from here: https://www.npmjs.com/package/serve.
diff --git a/index.html b/index.html
index ca3094a..be7c493 100644
--- a/index.html
+++ b/index.html
@@ -23,7 +23,7 @@ html {
 <body>
 	<div id="root">
 		<script type="module">
-			import init, { getState, openField } from "./pkg/rust_wasm_minesweeper.js";
+			import init, { getState, openField, toggleFlag } from "./pkg/rust_wasm_minesweeper.js";
 
 			async function main() {
 				await init();
@@ -50,6 +50,12 @@ html {
 							openField(x, y);
 							render();
 						});
+
+						elem.addEventListener("contextmenu", e => {
+							e.preventDefault();
+							toggleFlag(x, y);
+							render();
+						});
 						root.appendChild(elem);
 					}
 				}
diff --git a/src/lib.rs b/src/lib.rs
index f6fc4ae..259f55e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -19,3 +19,8 @@ pub fn get_state() -> String {
 pub fn open_field(x: usize, y: usize) {
 	MINESWEEPER.with(move |ms| ms.borrow_mut().open((x, y)));
 }
+
+#[wasm_bindgen(js_name = toggleFlag)]
+pub fn toggle_flag(x: usize, y: usize) {
+    MINESWEEPER.with(|ms| ms.borrow_mut().toggle_flag((x, y)));
+}
diff --git a/src/minesweeper.rs b/src/minesweeper.rs
index 81426c4..0897cf0 100644
--- a/src/minesweeper.rs
+++ b/src/minesweeper.rs
@@ -75,8 +75,29 @@ impl Minesweeper {
 			.count() as u8
 	}
 
+    pub fn count_flags(&self, position: Position) -> u8 {
+		self.iter_neighbours(position)
+			.filter(|pos| self.flags.contains(pos))
+			.count() as u8
+    }
+
 	pub fn open(&mut self, position: Position) -> Option<OpenResult> {
-		if self.game_over || self.open_fields.contains(&position) || self.flags.contains(&position) {
+        if self.open_fields.contains(&position) {
+			let mines = self.count_mines(position);
+            let flags = self.count_flags(position);
+
+            if mines == flags {
+				for neighbour in self.iter_neighbours(position) {
+                    if !self.flags.contains(&neighbour) && !self.open_fields.contains(&neighbour) {
+                        self.open(neighbour);
+                    }
+				}
+            }
+
+			return None;
+        }
+
+		if self.game_over || self.flags.contains(&position) {
 			return None;
 		}
 
@@ -89,7 +110,9 @@ impl Minesweeper {
 			let mines = self.count_mines(position);
 			if mines == 0 {
 				for neighbour in self.iter_neighbours(position) {
-					self.open(neighbour);
+                    if !self.open_fields.contains(&neighbour) {
+                        self.open(neighbour);
+                    }
 				}
 			}
 			Some(OpenResult::NoMine(mines))
