diff --git a/mockup/mockup.py b/mockup/mockup.py
index 19eef9d..ba9ff89 100644
--- a/mockup/mockup.py
+++ b/mockup/mockup.py
@@ -6,12 +6,31 @@
 
 # Simple example: Concrete types
 def add_one(number: int) -> int:
+    """
+    Add 1 to an `int`, returning the sum.
+
+    >>> add_one(9)
+    10
+    >>> add_one(-11)
+    -10
+    >>> add_one(2**63-1)
+    9223372036854775808
+    """
     return number + 1
 
 
 # Union types.
 # Optional[T] == Union[T, None]
 def reciprocal(number: Union[int, float]) -> Optional[float]:
+    """
+    Return the reciprocal of the given number. If the number is zero, return `None`.
+
+    >>> reciprocal(5)
+    0.2
+    >>> reciprocal(-20)
+    -0.05
+    >>> reciprocal(-0.0) # returns None
+    """
     if number == 0:
         return None
     return 1 / number
@@ -19,6 +38,15 @@ def reciprocal(number: Union[int, float]) -> Optional[float]:
 
 # Abstract types
 def flatten_ints(its: Iterable[Iterable[int]]) -> Iterable[int]:
+    """
+    Given an iterable of iterables of ints, return an iterable of all the ints
+    in the inner iterables.
+
+    >>> list(flatten_ints([[9, 11], [12], [4, 5]]))
+    [9, 11, 12, 4, 5]
+    >>> list(flatten_ints([[], (), set()]))
+    []
+    """
     for it in its:
         for i in it:
             yield i
@@ -26,13 +54,32 @@ def flatten_ints(its: Iterable[Iterable[int]]) -> Iterable[int]:
 
 # Types generic over a TypeVar
 def flatten_generic(its: Iterable[Iterable[T]]) -> Iterable[T]:
+    """
+    Given an iterable of iterables, return an iterable of all the inner
+    elements in the inner iterables.
+
+    >>> list(flatten_ints(["hi", (4, 2.54)]))
+    ['h', 'i', 4, 2.54]
+    """
     for it in its:
         for i in it:
             yield i
 
 
 class Circle:
+    """
+    Circle(radius) -> Self
+
+    A `Circle` represents the abstract geometric shape.
+
+    >>> c = Circle(2.21); c.diameter
+    4.42
+    >>> c2 = Circle.from_circumference(100); round(c2.radius, 3)
+    15.916
+    """
+
     PI = 3.14159
+
     __slots__ = ["radius"]
 
     def __init__(self, radius: Union[int, float]):
diff --git a/pytest.ini b/pytest.ini
new file mode 100644
index 0000000..df3eb51
--- /dev/null
+++ b/pytest.ini
@@ -0,0 +1,2 @@
+[pytest]
+addopts = --doctest-modules