What we are making: no computers at all this session. You are going to program a person, and find out why it is harder than it sounds.

The idea

A computer does exactly what you say. Not what you meant. It never guesses, never fills in the obvious bit, and never says "you probably meant left".

That is the single most important thing to understand this year, and the fastest way to understand it is to be on the receiving end of bad instructions.

Do it — the sandwich

Your teacher has bread, and something to put on it. One of you writes instructions. Your teacher follows them exactly.

1. In your team, write down how to make the sandwich. Numbered steps.
2. Hand it over. Say nothing while it is read.
3. Watch. Your teacher will follow what you wrote, not what you meant.
4. Write down every place it went wrong.
5. Fix your instructions. Try again.

Do it — the robot walk

In pairs, in the corridor or an open space. One person is the robot, one is the programmer. The robot's eyes are closed.

• The robot understands exactly four instructions: forward one step, turn left, turn right, stop.
• The robot does nothing else. No interpreting, no avoiding furniture, no helping.
• The programmer writes the whole list before the robot starts moving. No shouting corrections.

1. Pick a target across the room — a chair, a door.
2. Write the full list of instructions.
3. Run it. Do not speak once it has started.
4. Note where it actually ended up.
5. Swap roles and do it again.

Try it — order matters

These four steps are all correct. In this order they do not work. Rewrite them so they do.

What you learned

• A computer does what you say, not what you mean.
• Your brain hides the obvious steps. Write them down anyway.
• Order changes everything.
• A bug is a mistake in your instructions, not a disaster.
• Debugging is normal. It is most of the job.