diff --git a/interfaces.c b/interfaces.c
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+++ b/interfaces.c
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+#include <math.h>
+#include <stdio.h>
+
+// Here's a basic interface for geometric shapes.
+typedef struct {
+    double (*area)(void *);
+    double (*perim)(void *);
+} geometry;
+
+// For our example we'll implement this interface on
+// `rect` and `circle` types.
+typedef struct {
+    double width, height;
+} rect;
+
+typedef struct {
+    double radius;
+} circle;
+
+// To implement an interface in C, we need to define functions
+// that match the function pointers in the interface.
+// Here we implement `geometry` on `rect`s.
+double rect_area(void *r) {
+    rect *rect_ptr = (rect *)r;
+    return rect_ptr->width * rect_ptr->height;
+}
+
+double rect_perim(void *r) {
+    rect *rect_ptr = (rect *)r;
+    return 2 * rect_ptr->width + 2 * rect_ptr->height;
+}
+
+// The implementation for `circle`s.
+double circle_area(void *c) {
+    circle *circle_ptr = (circle *)c;
+    return M_PI * circle_ptr->radius * circle_ptr->radius;
+}
+
+double circle_perim(void *c) {
+    circle *circle_ptr = (circle *)c;
+    return 2 * M_PI * circle_ptr->radius;
+}
+
+// If a variable has an interface type, then we can call
+// methods that are in the named interface. Here's a
+// generic `measure` function taking advantage of this
+// to work on any `geometry`.
+void measure(void *g, geometry *geom) {
+    printf("Area: %f\n", geom->area(g));
+    printf("Perimeter: %f\n", geom->perim(g));
+}
+
+int main() {
+    rect   r = {.width = 3, .height = 4};
+    circle c = {.radius = 5};
+
+    // The `circle` and `rect` struct types both
+    // implement the `geometry` interface so we can use
+    // instances of these structs as arguments to `measure`.
+    geometry rect_geometry = {rect_area, rect_perim};
+    geometry circle_geometry = {circle_area, circle_perim};
+
+    printf("Rectangle:\n");
+    measure(&r, &rect_geometry);
+
+    printf("\nCircle:\n");
+    measure(&c, &circle_geometry);
+
+    return 0;
+}