diff --git a/LearnEnglish/LearnEnglish/Models/NumberWords.swift b/LearnEnglish/LearnEnglish/Models/NumberWords.swift
index d286d95..d209e05 100644
--- a/LearnEnglish/LearnEnglish/Models/NumberWords.swift
+++ b/LearnEnglish/LearnEnglish/Models/NumberWords.swift
@@ -151,7 +151,7 @@ enum NumberWords {
 
     static func randomWord() -> Word {
         let n = Int.random(in: 0...999_999_999)
-        return Word(word: english(n),
+        return Word(categoryId: "numbers", word: english(n),
                     phonetic: "/\(phonetic(n))/",
                     meaning: chinese(n),
                     example: "The number is \(english(n)).",
@@ -163,7 +163,7 @@ enum NumberWords {
     }
 
     private static func makeWord(_ n: Int) -> Word {
-        Word(word: english(n),
+        Word(categoryId: "numbers", word: english(n),
              phonetic: "/\(phonetic(n))/",
              meaning: chinese(n),
              example: "I can count to \(english(n)).",
diff --git a/LearnEnglish/LearnEnglish/Models/Word.swift b/LearnEnglish/LearnEnglish/Models/Word.swift
index b52e4d8..bd68510 100644
--- a/LearnEnglish/LearnEnglish/Models/Word.swift
+++ b/LearnEnglish/LearnEnglish/Models/Word.swift
@@ -13,7 +13,8 @@ struct WordCategory: Codable, Identifiable, Hashable {
 }
 
 struct Word: Codable, Identifiable, Hashable {
-    var id: String { word }
+    var id: String { "\(categoryId)/\(word)" }
+    var categoryId: String = ""
     let word: String
     let phonetic: String
     let meaning: String
diff --git a/LearnEnglish/LearnEnglish/Services/WordStore.swift b/LearnEnglish/LearnEnglish/Services/WordStore.swift
index dac8d9a..6a8df25 100644
--- a/LearnEnglish/LearnEnglish/Services/WordStore.swift
+++ b/LearnEnglish/LearnEnglish/Services/WordStore.swift
@@ -28,10 +28,20 @@ final class WordStore {
             let data = try Data(contentsOf: url)
             let decoded = try JSONDecoder().decode(WordBank.self, from: data)
             let categories = decoded.categories.map { category -> WordCategory in
-                guard category.id == "numbers" else { return category }
-                var words = NumberWords.makeWords(atoms: category.words)
-                let extras = category.words.filter { NumberWords.atomNumbers[$0.word] == nil }
-                words.append(contentsOf: extras)
+                let tagged: [Word]
+                if category.id == "numbers" {
+                    var words = NumberWords.makeWords(atoms: category.words)
+                    let extras = category.words.filter { NumberWords.atomNumbers[$0.word] == nil }
+                    words.append(contentsOf: extras)
+                    tagged = words
+                } else {
+                    tagged = category.words
+                }
+                let words = tagged.map { word -> Word in
+                    var w = word
+                    w.categoryId = category.id
+                    return w
+                }
                 return WordCategory(id: category.id, name: category.name, icon: category.icon,
                                     words: words)
             }
diff --git a/LearnEnglish/LearnEnglish/Views/QuizViewController.swift b/LearnEnglish/LearnEnglish/Views/QuizViewController.swift
index f8e65c2..1d96773 100644
--- a/LearnEnglish/LearnEnglish/Views/QuizViewController.swift
+++ b/LearnEnglish/LearnEnglish/Views/QuizViewController.swift
@@ -52,8 +52,8 @@ final class QuizViewController: UIViewController {
         self.isUppercase = uppercase
         let words = config.words.map { word in
             let text = uppercase ? word.word.uppercased() : word.word.lowercased()
-            return Word(word: text, phonetic: word.phonetic, meaning: word.meaning,
-                        example: word.example, exampleMeaning: word.exampleMeaning)
+            return Word(categoryId: word.categoryId, word: text, phonetic: word.phonetic,
+                        meaning: word.meaning, example: word.example, exampleMeaning: word.exampleMeaning)
         }
         self.questions = QuizGenerator.makeQuestions(mode: config.mode, words: words)
         super.init(nibName: nil, bundle: nil)
@@ -509,8 +509,8 @@ final class QuizViewController: UIViewController {
         view.subviews.compactMap { $0 as? QuizResultView }.forEach { $0.removeFromSuperview() }
         let words = config.words.map { word in
             let text = isUppercase ? word.word.uppercased() : word.word.lowercased()
-            return Word(word: text, phonetic: word.phonetic, meaning: word.meaning,
-                        example: word.example, exampleMeaning: word.exampleMeaning)
+            return Word(categoryId: word.categoryId, word: text, phonetic: word.phonetic,
+                        meaning: word.meaning, example: word.example, exampleMeaning: word.exampleMeaning)
         }
         questions = QuizGenerator.makeQuestions(mode: config.mode, words: words)
         currentIndex = 0