She is expected to make a full recovery.

Mrs Wu, who is from Guangdong, China, moved to Hong Kong about six months ago.

She works part-time in an old folks' home.

The police have not confirmed if someone threw the screwdriver down deliberately or if it fell due to an accident. They are still investigating.

An engineer from the Hong Kong Polytechnic University's mechanical engineering department, Mr Lu Jueqiang, said that the woman was extremely fortunate to escape being hit by what would have become a gigantic flying dart.

He said that assuming the screwdriver weighs 75g, it would have attained a speed of more than 100kmh if it had been dropped from the 15th storey or higher.

Said Mr Lu: 'She's actually very lucky.

'If the screwdriver had pierced her head, she would have suffered very bad injuries to the brain - if she was not dead by then.'

This story was first published in The New Paper on Nov 20, 2008.