diff --git a/README.md b/README.md
index ec9bfbd..8d1fab1 100644
--- a/README.md
+++ b/README.md
@@ -121,6 +121,27 @@ Synthesizes a Python function described in the prompt.
 (3, 2)
 ```
 
+### `davinci_functions.explain`
+
+Describes the behavior of given functions.
+
+```python
+>>> def f(x):
+...     return x * 3
+... 
+>>> davinci_functions.explain(f)
+'This function takes a variable x and multiplies it by 3, then returns the result.'
+>>> def f(a, b, c):
+...     return (-b + math.sqrt(b**2 - 4.0 * a * c)) / (2.0 * a)
+... 
+>>> davinci_functions.explain(f)
+'This function implements the Quadratic Formula to calculate the solution of a ...
+ quadratic equation. The equation is of the form ax^2 + bx + c = 0. The function ...
+ takes three parameters a, b, and c, which are the coefficients of the equation. It ...
+ then calculates the solution using the formula (-b + sqrt(b^2 - 4ac)) / (2a) and ...
+ returns the result.'
+```
+
 
 ## Caveats
 
diff --git a/pyproject.toml b/pyproject.toml
index 5502aaa..ce46616 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,6 +28,7 @@ classifiers = [
     "Programming Language :: Python :: 3.11",
 ]
 dependencies = [
+    "dill>=0.3.6",
     "openai>=0.27.0",
 ]
 dynamic = [
diff --git a/src/davinci_functions/__init__.py b/src/davinci_functions/__init__.py
index 47d8d61..3032e34 100644
--- a/src/davinci_functions/__init__.py
+++ b/src/davinci_functions/__init__.py
@@ -9,11 +9,13 @@
 except Exception:
     __version__ = ""
 
+from davinci_functions._explain import explain
 from davinci_functions._function import function
 from davinci_functions._judge import judge
 from davinci_functions._list import list
 
 __all__ = [
+    "explain",
     "function",
     "judge",
     "list",
diff --git a/src/davinci_functions/_explain.py b/src/davinci_functions/_explain.py
new file mode 100644
index 0000000..1712f45
--- /dev/null
+++ b/src/davinci_functions/_explain.py
@@ -0,0 +1,68 @@
+"""implementation of `explain` function."""
+
+from __future__ import annotations
+
+import textwrap
+from collections.abc import Callable
+from typing import Any
+
+import dill  # type: ignore[import]
+import openai
+
+
+def explain(fn: Callable[..., Any]) -> str:
+    """Describes the behavior of the given function.
+
+    Args:
+        fn: A callable. The corresponding source code must be associated to this object.
+            i.e., inspect.getsource(fn) should work.
+
+    Returns:
+        The description of `fn` written by GPT.
+
+    Example:
+        >>> def my_function(x):
+        ...     if x == 0:
+        ...         return 1.0
+        ...     else:
+        ...         return math.sin(x) / x
+        ...
+        >>> f = davinci_functions.explain(my_function)
+        'This function takes a single argument x and returns a float value. If x is ...
+         0, it returns 1.0, otherwise it returns the result of the sine of x divided ...
+         by x. This is known as the sinc function.'
+    """
+    src = dill.source.getsource(fn)
+    base_prompt = textwrap.dedent(
+        """\
+        Describe the detailed behavior of the following Python function.
+        The question may have multiple lines.
+        It is written between "BEGIN_QUESTION" and "END_QUESTION".
+        The answer is written after "ANSWER".
+        The result may have multiple lines.
+        It is recommended to write as detailed explanation as possible.
+        If the algorithm written in the function has some contexts, describe it as well.
+        For example, if the algorithm has a name, it should be written in the answer.
+
+        BEGIN_QUESTION
+        def sum(a, b):
+            return a + b
+        END_QUESTION
+
+        ANSWER
+        This function adds two variables a and b and returns its result.
+
+        BEGIN_QUESTION
+        {}
+        END_QUESTION
+
+        ANSWER
+        """
+    )
+
+    return openai.Completion.create(  # type: ignore[no-any-return, no-untyped-call]
+        model="text-davinci-003",
+        prompt=base_prompt.format(src),
+        max_tokens=2048,
+        temperature=0,
+    )["choices"][0]["text"]
diff --git a/src/davinci_functions/_explain_test.py b/src/davinci_functions/_explain_test.py
new file mode 100644
index 0000000..f2f60b7
--- /dev/null
+++ b/src/davinci_functions/_explain_test.py
@@ -0,0 +1,19 @@
+"""Tests for davinci_functions._explain."""
+
+from __future__ import annotations
+
+import pytest_mock
+
+from davinci_functions._explain import explain
+
+
+def test_explain(mocker: pytest_mock.MockerFixture) -> None:
+    mocker.patch(
+        "openai.Completion.create",
+        return_value={"choices": [{"text": "This is a test."}]},
+    )
+
+    def dummy() -> int:
+        return 42
+
+    assert explain(dummy) == "This is a test."
diff --git a/src/davinci_functions/_function.py b/src/davinci_functions/_function.py
index 699ed2b..15ad98a 100644
--- a/src/davinci_functions/_function.py
+++ b/src/davinci_functions/_function.py
@@ -1,4 +1,4 @@
-"""implementation of `list` function."""
+"""implementation of `function` function."""
 
 from __future__ import annotations
 
@@ -24,7 +24,8 @@ def function(prompt: str) -> Callable[..., Any]:
         A callable object that may represent the behavior of the prompt.
 
     Raises:
-        SyntaxError: GPT didn't return a Python literal.
+        SyntaxError: GPT didn't return a meaningful Python program.
+        TypeError: GPT didn't return a Python callable.
 
     Example:
         >>> f = davinci_functions.list("Multiply the argument x by 2.")
diff --git a/src/davinci_functions/_function_test.py b/src/davinci_functions/_function_test.py
index 629458f..7a49306 100644
--- a/src/davinci_functions/_function_test.py
+++ b/src/davinci_functions/_function_test.py
@@ -8,7 +8,7 @@
 from davinci_functions._function import function
 
 
-def test_judge_true(mocker: pytest_mock.MockerFixture) -> None:
+def test_function(mocker: pytest_mock.MockerFixture) -> None:
     mocker.patch(
         "openai.Completion.create",
         return_value={"choices": [{"text": "def func(x):\n    return x * 2"}]},
@@ -19,7 +19,7 @@ def test_judge_true(mocker: pytest_mock.MockerFixture) -> None:
     assert f(3) == 6
 
 
-def test_judge_non_python(mocker: pytest_mock.MockerFixture) -> None:
+def test_function_non_python(mocker: pytest_mock.MockerFixture) -> None:
     mocker.patch(
         "openai.Completion.create",
         return_value={"choices": [{"text": "Error."}]},
@@ -29,7 +29,7 @@ def test_judge_non_python(mocker: pytest_mock.MockerFixture) -> None:
         function("This is a test.")
 
 
-def test_judge_non_function(mocker: pytest_mock.MockerFixture) -> None:
+def test_function_non_function(mocker: pytest_mock.MockerFixture) -> None:
     mocker.patch(
         "openai.Completion.create",
         return_value={"choices": [{"text": "func = 42"}]},