Ondo is a unique acoustic thermometer that allows you to measure the temperature of your room using your iPhone or iPad and a standard Apple headset. Developed by SCB Associates Ltd, this experimental app uses the principle that the speed of sound in air changes with temperature, and by measuring the speed of sound, it can calculate the air temperature.

To use Ondo, you need to build your headset thermometer using an old credit or debit card, four elastic bands, and the headset. Once you've built it, follow the instructions in the app or on the website to calibrate it to the starting temperature, and you're ready to go. You can then move the phone and headset to a different temperature zone, and watch the reading change.

Ondo has a temperature range of -40 to 50 degrees Celsius and an accuracy of +/- 1 degree, dependent upon headset fixing. It is essential to note that this app will only work with a headset connected and set up according to the instructions in the app. Do not buy this app if you do not have a suitable headset, as it will not work with AirPods.