Part of a ruler marked in centimetres and millimetres. The centimetre marks are labelled 1, 2, 3.
There is a red marker at the left-hand end of the ruler.
When you drag the marker to the half centimetre mark, you get correct feedback.
If you drag the marker to the wrong place, you get incorrect feedback.
After two wrong attempts, you can click on a 'Show me' button to see the correct answer.