diff --git a/day21.py b/day21.py
new file mode 100644
index 0000000..d53cb6e
--- /dev/null
+++ b/day21.py
@@ -0,0 +1,86 @@
+import collections
+
+
+def read_food():
+    with open('day21/input') as reader:
+        return reader.readlines()
+
+
+food_as_text = read_food()
+
+food = []
+for f in food_as_text:
+    ingredients, allergens = f[:-1].split(' (contains ')
+    food.append((ingredients.split(' '), allergens[:-1].split(', ')))
+
+
+def get_allergen_possibilities(food):
+    allergen_possibilities = {}
+    for ingredients, allergens in food:
+        for a in allergens:
+            if a not in allergen_possibilities.keys():
+                allergen_possibilities[a] = set(ingredients)
+            else:
+                allergen_possibilities[a] = allergen_possibilities[a].intersection(set(ingredients))
+    return allergen_possibilities
+
+
+def get_all_ingredients_with_possible(allergen_possibilities):
+    all_ingredients_with_possible = set()
+    for i in allergen_possibilities.values():
+        all_ingredients_with_possible = all_ingredients_with_possible.union(i)
+    return all_ingredients_with_possible
+
+
+def get_all_ingredients(food):
+    all_ingredients = set()
+    for ingredients, allergens in food:
+        all_ingredients = all_ingredients.union(ingredients)
+    return all_ingredients
+
+
+def count_no_allergens(food, no_allergen):
+    c = 0
+    for ingredients, allergens in food:
+        for i in ingredients:
+            if i in no_allergen:
+                c += 1
+    return c
+
+
+def find_allergens(allergen_possibilities):
+    defined_allergens = {}
+    while True:
+        found_key, found_value = next((key, value) for key, value in allergen_possibilities.items() if len(value) == 1)
+        ingredient_defined = list(found_value)[0]
+
+        defined_allergens[found_key] = ingredient_defined
+
+        del allergen_possibilities[found_key]
+        for allergen in allergen_possibilities.keys():
+            if ingredient_defined in allergen_possibilities[allergen]:
+                allergen_possibilities[allergen].remove(ingredient_defined)
+
+        if not allergen_possibilities:
+            break
+
+    return defined_allergens
+
+
+allergen_possibilities = get_allergen_possibilities(food)
+all_ingredients_with_possible = get_all_ingredients_with_possible(allergen_possibilities)
+all_ingredients = get_all_ingredients(food)
+
+no_allergen = all_ingredients.difference(all_ingredients_with_possible)
+
+res = count_no_allergens(food, no_allergen)
+
+print(res)
+
+defined_allergens = find_allergens(allergen_possibilities)
+
+ordered_by_insertion = collections.OrderedDict(sorted(defined_allergens.items()))
+
+canonical_dangerous_ingredient_list = ','.join([v for v in ordered_by_insertion.values()])
+
+print(canonical_dangerous_ingredient_list)