It falls straight down, and only travels diagonally when something is blocking its path directly below. That makes the route predictable, so you can trace where a chunk will end up before you melt it.

It freezes solid where it lands and becomes an obstacle for everything you melt afterwards. Stray drips are the main reason levels become unsolvable, which is why order matters so much.

Because melting the wrong chunk first sends liquid into a place that is not ready for it, and the frozen result blocks a channel a later chunk needed. Most levels require you to melt everything — the puzzle is working out the sequence.

Use Reset and start the sequence again. A board that has frozen shut is usually quicker to restart than to work around, and Hint is there if the intended order is not obvious.