Look at this thermometer. It is reading −4 °C. What does the minus sign in front of the 4 tell us about how cold it is outside?

The interactive shows three number lines from −10 to +10, each with markers to read. Look at how far each marker sits from zero. For the lift and the temperature, ask yourself: does this change cross zero or stay on one side?

Today we place these markers on the −10 to +10 line together, one at a time: first −6, then −9, then +4, and finally 0. Then one pupil places it and we check together before moving to the next.

Today we work through these temperature-change problems together, each a little trickier than the last. The freezer falls from −16 °C to −19 °C, staying below zero the whole time. Mullingar drops from +3 °C to −4 °C in a cold snap, crossing over zero. The carpark goes from +1 °C to −6 °C, crossing zero too. Then the stretch, which also crosses zero the other way: Waterford rises from −4 °C to +3 °C as the sun comes up.