Prudent hikers carry lots of water with them, particularly on a hot day. Hiking up Tse Wan Shan 慈雲山 (Temple Hill), however, I saw many people carrying empty plastic bottles uphill.
It turns out there are many places up the hill where streams of water run off the hill. There is one place near the Kwun Yin Temple 觀音廟. which is particularly popular among the water collectors.
This temple is the reason why Tse Wan Shan is also called Temple Hill. In fact, "Tse Wan 慈雲" itself refers to the “cloud of compassion” of Kwun Yin (觀音的慈雲). That's probably why this particular stream behind the temple is so popular.