HUNDREDS of girls in different coloured uniforms recently marched into the Hong Kong International School hall with only one thought on their minds - sister Guides and Scouts around the world. ''Thinking Day'' on February 22 was an annual event of the World Association of Girl Guides and Girl Scouts. Members came together recently and celebrated the birthday of Lord Baden-Powell, the founder of the Scouting movement. Meanwhile, it was a day for them to think about other members in the organisation. ''The girls are supposed to consider what girl scouting means to them and more importantly, perhaps, is thinking of all the Scouts around the world because Girl Scouts are taught they are sisters to every other Girl Scouts, which makes it a very international event,'' said Laura Vinogradov, the Special Events Co-ordinator of Girl Scouts USA. This was the first time local Guides and Girl Scouts USA celebrated the event together. About 170 Girl Scouts and 200 Guides, aged between 51/2 and 15, were present at the ceremony. ''Things like this go on all over the world, but it's unusual that you have a situation where the girls are actually there and they can meet each other,'' said Laura. ''We want the girls to see the global picture of girl scouting.'' A traditional pin, the world association pin, was given to each girl after their first Thinking Day in order to remind them they belonged to a world association.' Apart from thinking, ''enjoyment'' and ''mixing'' were very much emphasised on the day, said Christine Chiu, the Woodland District Commissioner of the English-speaking Hong Kong Girl Guides. The girls had an enjoyable evening. They designed and stitched their own flags for their troops. They sang and danced as well as exchanged gifts. The evening ended with the girls assembled in the middle of the hall singing ''Make New Friends'' hand-in-hand. They then marched out and each of them picked a wish from the wishing well which their sister Scouts have put a wish in for their new friends. ''My wish is for a wonderful life and good health,'' said Casey Gallagher, a member of the Girl Scouts' Brownies cheerfully.