diff --git a/go/02-next-level/01-interfaces.go b/go/02-next-level/01-interfaces.go
new file mode 100644
index 0000000..6a6e25f
--- /dev/null
+++ b/go/02-next-level/01-interfaces.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type geometry interface {
+	area() float64
+	perim() float64
+}
+
+type rect struct {
+	width, height float64
+}
+
+var _ geometry = rect{}
+
+type circle struct {
+	radius float64
+}
+
+var _ geometry = circle{}
+
+func (r rect) area() float64 {
+	return r.width * r.height
+}
+func (r rect) perim() float64 {
+	return 2*r.width + 2*r.height
+}
+
+func (c circle) area() float64 {
+	return math.Pi * c.radius * c.radius
+}
+func (c circle) perim() float64 {
+	return 2 * math.Pi * c.radius
+}
+
+func measure(g geometry) {
+	fmt.Println(g)
+	fmt.Println(g.area())
+	fmt.Println(g.perim())
+}
+
+func detectCircle(g geometry) {
+	if c, ok := g.(circle); ok {
+		fmt.Println("circle with radius", c.radius)
+	}
+}
+
+func main() {
+	r := rect{width: 3, height: 4}
+	c := circle{radius: 5}
+
+	measure(r)
+	measure(c)
+
+	detectCircle(r)
+	detectCircle(c)
+}