Wu Chang, a young teacher with the Paotongshu Primary School in Chengdu, Sichuan province, used to hate taking the subway because he was always asked to drink water when he wasn't thirsty.

"I like bringing bottled water with me. In a subway station, a security inspector would ask me to open the bottle and have a drink. I didn't feel well," he said.

Passengers like Wu will be happy to find inspectors will no longer force them to drink thanks to the introduction of handheld liquid detectors in all the subway security checkpoints in the city.

The detector will show whether the liquid is safe or not immediately after the small machine, weighing less than 500 grams and about 25 centimeters long, nears the liquid, according to the West China Metropolis Daily.

The detector sends a microwave that passes through the liquid. Through analyzing the data reflected by the microwave, the detector can determine whether the liquid is safe. If it is not, an alarm will sound, a security inspector told the newspaper.

The detector is sensitive to inflammable and explosive liquids, such as petroleum. But it can check only materials contained in glass and plastic bottles and not metals.