Rotating Triangle is a task on the NRICH website.
One approach to geometric proof is to focus on logical arguments that build on pupils' prior knowledge in order to demonstrate the truth of some geometrical result. Preferably, the result should be one that has some element of surprise and is based around something previously conjectured by pupil after conducting a well-chosen ‘experiment’.
This task provides learners with an opportunity to make a conjecture and then to test whether their conjecture holds. This leads to the opportunity to try to prove the result.