You can type the mathematical constant pi (π) into an Android text message by using the following steps:

- Open the keyboard on your Android device.
- Make sure that you have the Google Keyboard selected.
- Touch the "?123" button on the lower left.
- Touch the "=<" button on the lower left.
- In the top row, you should find the mathematical symbol "π".

(Aria did not provide this answer, but should include it.)