diff --git a/2020/day21.py b/2020/day21.py
new file mode 100644
index 0000000..0960933
--- /dev/null
+++ b/2020/day21.py
@@ -0,0 +1,43 @@
+import re
+import sys
+from functools import reduce
+
+LINE_RE = re.compile(r"(.+) \(contains (.+)\)")
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+    foods = []
+    for line in lines:
+        i, a = LINE_RE.match(line).groups()
+        foods.append((set(i.split()), set(a.split(", "))))
+    ingredients = set(i for il, _ in foods for i in il)
+    allergens = set(a for _, al in foods for a in al)
+
+    # Part 1
+    matches = {}
+    while len(matches) < len(allergens):
+        for allergen in allergens:
+            p_ings = [
+                set(filter(lambda i: i not in matches, f[0]))
+                for f in foods if allergen in f[1]
+            ]
+            c_ings = reduce(lambda a, b: a & b, p_ings, ingredients)
+            if len(c_ings) == 1:
+                i = c_ings.pop()
+                matches[i] = allergen
+    clean_ings = ingredients.difference(set(matches.keys()))
+    num_appearances = sum(
+        ci in fis
+        for fis, _ in foods
+        for ci in clean_ings
+    )
+    print(f"Clean ingredients appear {num_appearances} times.")
+
+    # Part 2
+    canonical = ",".join(
+        f for f, a in sorted(matches.items(), key=lambda m: m[1])
+    )
+    print("Canonical list:", canonical)
+
+if __name__ == "__main__":
+    main()