Question
What is the reason for resorting to Wudu and Tayammum in the case of water whose purity is in doubt? Why is Tayammum alone not sufficient?
Answer
I say, and with God's help: it is possible that it is pure, so the tayammum is not valid, and it is possible that it is impure, so tayammum is necessary. Therefore, it is essential to combine ablution and tayammum, and God knows best.