While Stephen Hawking was programming the voice synthesiser attached to his computer to answer the first question at yesterday's press conference, one of his assistants explained how it worked. He said an infra-red device controlled by Professor Hawking's blinking sent a single on/off signal to the computer mounted on his wheelchair. 'The software interprets this and highlights the first row [of words on the computer screen] for half a second and then the second row, and so on.' When Professor Hawking gets to the row he wants, he blinks the switch to select columns on it, and finally the item or word he seeks. 'So if we wanted 'hello', he would choose 'H' and it would display all the words starting with H on a series of pages.' He then scrolls down the pages and across to 'hello'. At that point, the word can be sent to the speech synthesiser or saved to be used as part of a sentence. The Hong Kong Press Photographers' Association yesterday apologised for the use of flash when Professor Hawking arrived at the airport. Flash interferes with his computer system and photographers had been asked not to use it.