Block coding lesson 4: Finding and fixing bugs
UK version. UK school years and UK spelling. · US version of this lesson
A bug is a mistake in a program. Debugging means finding it and fixing it. A complete 30-minute block coding lesson (Years 1–2, KS1), from Unit 2: Bugs and walls. Myrtle the Turtle runs right here in your browser – nothing to install.
- Time
- 30 minutes + 10 min homework
- Year
- Years 1–2, KS1 (ages 5–7)
- You need
- A computer or tablet, squared paper and a pencil
- Marked
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know anything about coding. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: watch Myrtle first, then have a go. Programs are made from blocks – no typing – and every answer is marked straight away.
- Before the screen: Write a "buggy" route on paper for a grown-up and ask them to find your bug.
- Press Start the lesson below and watch the worked examples together. Press Run and say what each block does before Myrtle does it.
- Let your child do the practice on their own. When Myrtle goes the wrong way, that's a clue, not a failure – press Run again and watch which block it is on.
- Finish with the talk question: “How did you find the bug?”
- Later that day or the next day: the homework (10 minutes), then a practice test.
Watch out for
- Starting the whole program again from nothing. Most bugs are one wrong block. Find where it first goes wrong.
- Thinking a bug means you are bad at coding. Every programmer makes bugs. Finding them is part of the job.
Miss Lovelace's tip for home: ask your child to be Myrtle. Read out the blocks and let them walk it on the floor. If they can act it out, they understand it.
What your child will learn
We are learning to find the block that makes a program go wrong.
Success looks like
- I can find the block that makes a program go wrong.
- I can fix a bug by changing one block.
- I can test again after a fix.
Key words
bug · debug · test · fix
The big ideas (for the adult)
A bug is a mistake in a program. Debugging means finding it and fixing it.
Run the program and watch: the bug is usually at the first moment Myrtle does something we did not expect.
A worked example
move forward 2 turn left move forward 4 turn right move forward 1
Press Start the lesson to watch the turtle run it.
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter | 4 min | Marked by Classfern |
| Teach | 7 min | Watch, talk and try |
| Worked example and try it | 6 min | Marked by Classfern |
| Practice | 9 min | Marked by Classfern |
| Wrap up | 4 min | Marked by Classfern + self-check |
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.
Homework
10 minutes: 4 questions – work out what Myrtle does, then build or fix a program – and a short reflection. New questions every time, all marked instantly. Away from the screen: write a "buggy" route on paper for a grown-up and ask them to find your bug.
Practice tests
Curriculum
National curriculum in England, computing, key stage 1: pupils should be taught to create and debug simple programs; and to use logical reasoning to predict the behaviour of simple programs.
Contains public sector information licensed under the Open Government Licence v3.0.
Our lessons, blocks and questions are our own. They are not written or endorsed by any government department, testing agency or exam board.