diff --git a/projects/interfaces/buffer.go b/projects/interfaces/buffer.go
new file mode 100644
index 000000000..803f5808a
--- /dev/null
+++ b/projects/interfaces/buffer.go
@@ -0,0 +1,33 @@
+package main
+
+type OurByteBuffer struct {
+	bytes        []byte
+	readPosition int
+}
+
+func NewBufferString(s string) OurByteBuffer {
+	return OurByteBuffer{
+		bytes:        []byte(s),
+		readPosition: 0,
+	}
+}
+
+func (b *OurByteBuffer) Bytes() []byte {
+	return b.bytes
+}
+
+func (b *OurByteBuffer) Write(bytes []byte) (int, error) {
+	b.bytes = append(b.bytes, bytes...)
+	return len(bytes), nil
+}
+
+func (b *OurByteBuffer) Read(to []byte) (int, error) {
+	remainingBytes := len(b.bytes) - b.readPosition
+	bytesToRead := len(to)
+	if remainingBytes < bytesToRead {
+		bytesToRead = remainingBytes
+	}
+	copy(to, b.bytes[b.readPosition:b.readPosition+bytesToRead])
+	b.readPosition += bytesToRead
+	return bytesToRead, nil
+}
diff --git a/projects/interfaces/buffer_test.go b/projects/interfaces/buffer_test.go
new file mode 100644
index 000000000..30f94ac5a
--- /dev/null
+++ b/projects/interfaces/buffer_test.go
@@ -0,0 +1,57 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInitialBytesAreRead(t *testing.T) {
+	want := []byte("hello")
+
+	b := NewBufferString("hello")
+
+	got := b.Bytes()
+
+	require.Equal(t, want, got)
+}
+
+func TestSubsequentWritesAreAppended(t *testing.T) {
+	want := []byte("hello world")
+
+	b := NewBufferString("hello")
+
+	_, err := b.Write([]byte(" world"))
+	require.NoError(t, err)
+
+	got := b.Bytes()
+
+	require.Equal(t, want, got)
+}
+
+func TestReadWithSliceBigEnoughForWholeBuffer(t *testing.T) {
+	b := NewBufferString("hello world")
+
+	slice := make([]byte, 50)
+
+	n, err := b.Read(slice)
+	require.NoError(t, err)
+	require.Equal(t, 11, n)
+	require.Equal(t, []byte("hello world"), slice[:n])
+}
+
+func TestReadWithSliceSmallerThanWholeBuffer(t *testing.T) {
+	b := NewBufferString("hello world")
+
+	slice := make([]byte, 6)
+
+	n, err := b.Read(slice)
+	require.NoError(t, err)
+	require.Equal(t, 6, n)
+	require.Equal(t, []byte("hello "), slice)
+
+	n, err = b.Read(slice)
+	require.NoError(t, err)
+	require.Equal(t, 5, n)
+	require.Equal(t, []byte("world"), slice[:n])
+}
diff --git a/projects/interfaces/filtering_pipe.go b/projects/interfaces/filtering_pipe.go
new file mode 100644
index 000000000..74280f14c
--- /dev/null
+++ b/projects/interfaces/filtering_pipe.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"io"
+)
+
+type FilteringPipe struct {
+	writer io.Writer
+}
+
+func NewFilteringPipe(writer io.Writer) FilteringPipe {
+	return FilteringPipe{
+		writer: writer,
+	}
+}
+
+func (fp *FilteringPipe) Write(bytes []byte) (int, error) {
+	for i := range bytes {
+		if bytes[i] < '0' || bytes[i] > '9' {
+			if _, err := fp.writer.Write(bytes[i : i+1]); err != nil {
+				return i, err
+			}
+		}
+	}
+	// We return len(bytes) because io.Writer is documented to return how many bytes were processed, not how many were actually used.
+	return len(bytes), nil
+}
diff --git a/projects/interfaces/filtering_pipe_test.go b/projects/interfaces/filtering_pipe_test.go
new file mode 100644
index 000000000..097ad74f1
--- /dev/null
+++ b/projects/interfaces/filtering_pipe_test.go
@@ -0,0 +1,55 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteNoNumbers(t *testing.T) {
+	buf := bytes.NewBufferString("")
+
+	fp := NewFilteringPipe(buf)
+
+	n, err := fp.Write([]byte("hello"))
+	require.NoError(t, err)
+	require.Equal(t, 5, n)
+	require.Equal(t, "hello", buf.String())
+}
+
+func TestWriteJustNumbers(t *testing.T) {
+	buf := bytes.NewBufferString("")
+
+	fp := NewFilteringPipe(buf)
+
+	n, err := fp.Write([]byte("123"))
+	require.NoError(t, err)
+	require.Equal(t, 3, n)
+	require.Equal(t, "", buf.String())
+}
+
+func TestMultipleWrites(t *testing.T) {
+	buf := bytes.NewBufferString("")
+
+	fp := NewFilteringPipe(buf)
+
+	n, err := fp.Write([]byte("start="))
+	require.NoError(t, err)
+	require.Equal(t, 6, n)
+	n, err = fp.Write([]byte("1, end=10"))
+	require.NoError(t, err)
+	require.Equal(t, 9, n)
+	require.Equal(t, "start=, end=", buf.String())
+}
+
+func TestWriteMixedNumbersAndLetters(t *testing.T) {
+	buf := bytes.NewBufferString("")
+
+	fp := NewFilteringPipe(buf)
+
+	n, err := fp.Write([]byte("start=1, end=10"))
+	require.NoError(t, err)
+	require.Equal(t, 15, n)
+	require.Equal(t, "start=, end=", buf.String())
+}
diff --git a/projects/interfaces/go.mod b/projects/interfaces/go.mod
new file mode 100644
index 000000000..0687d0bba
--- /dev/null
+++ b/projects/interfaces/go.mod
@@ -0,0 +1,10 @@
+module github.com/CodeYourFuture/immersive-go-course/projects/interfaces
+
+go 1.19
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/stretchr/testify v1.8.2 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/projects/interfaces/go.sum b/projects/interfaces/go.sum
new file mode 100644
index 000000000..658c3d650
--- /dev/null
+++ b/projects/interfaces/go.sum
@@ -0,0 +1,16 @@
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+github.com/stretchr/testify v1.8.2 h1:+h33VjcLVPDHtOdpUCuF+7gSuG3yGIftsP1YvFihtJ8=
+github.com/stretchr/testify v1.8.2/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=