diff --git a/source/storage/common.ts b/source/storage/common.ts
new file mode 100644
index 0000000..581f122
--- /dev/null
+++ b/source/storage/common.ts
@@ -0,0 +1,33 @@
+import browser from "webextension-polyfill";
+import {createValue} from "@holllo/webextension-storage";
+
+export enum StorageKey {
+  IntroductionUnderstood = "introduction-understood",
+  ToursCompleted = "tours-completed",
+}
+
+export async function createIntroductionUnderstood() {
+  return createValue<boolean>({
+    deserialize: (input) => input === "true",
+    serialize: (input) => JSON.stringify(input),
+    key: StorageKey.IntroductionUnderstood,
+    storage: browser.storage.local,
+    value: false,
+  });
+}
+
+export async function createToursCompleted() {
+  return createValue<Set<TourId>>({
+    deserialize: (input) => new Set(JSON.parse(input) as TourId[]),
+    serialize: (input) => JSON.stringify(Array.from(input)),
+    key: StorageKey.ToursCompleted,
+    storage: browser.storage.local,
+    value: new Set([]),
+  });
+}
+
+export async function addCompletedTour(tourId: TourId): Promise<void> {
+  const toursCompleted = await createToursCompleted();
+  toursCompleted.value.add(tourId);
+  await toursCompleted.save();
+}