diff --git a/histogram.cpp b/histogram.cpp
index f66701b..24e81b0 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -2,7 +2,7 @@
 
 using namespace std;
 
-static void
+void
 find_minmax(const vector<double>& numbers, double& min, double& max) {
     min = numbers[0];
     max = numbers[0];
diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..34592d8
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,9 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+
+#include <vector>
+
+void
+find_minmax(const std::vector<double>& numbers, double& min, double& max)
+
+#endif // HISTOGRAM_INTERNAL_H_INCLUDED