A five-second habit before every move: tap the placement once before committing your full weight. A dull thunk means solid, a hollow ring means air behind it — route around those spots.
Ice changes quality within a metre, and the tap test is the cheapest information you get out there — five seconds, and it separates experienced ice climbers from beginners. The typical error is trusting the tool just because it stuck: an axe can sit cleanly in a hollow curtain and still bring a whole layer down with it. So add two more signals. Colour: deep blue to clear is dense and well-bonded, white or milky holds air and is weaker. Temperature: −5 to −10 °C is the plastic window, below −20 °C ice turns brittle, and direct sun makes it soft. What gets missed: the tap test does not make a route safe. Free-hanging icicles, columns, fresh debris and running water are turn-back signals no matter how they sound — and none of this replaces a course or a mountain guide.