"It's a Kipp generator," said Dick proudly, as he opened the brown parcel.

The strange instrument, rather like a glass snowman, glinted in the pale London afternoon sun on their little apartment balcony. Their mother had insisted that anything to do with sulphur had to be kept outdoors, winter or no winter.

"But Mother said this isn't our surprise. So what do you think the surprise is?" asked Dorothea.

"Oh, I expect it's whoever comes home for dinner with father," said Dick. "There were five dinner plates on the sideboard, and look behind you: the spare room door is open." Dorothea turned around. Yes, the glass door was hooked back on to the balcony railing. The inside door was shut to keep the rest of the flat warm. Mother was airing the spare room for a visitor.

"You'd make a good detective, Dick," said his sister.

Dick wasn't really listening. He was pouring a dark powder out of a folded sheet of paper into one of the round glass bulbs. "The hydrochloric goes on top. Pass me that bottle."

Dorothea was accustomed to being Dick's laboratory assistant, and she didn't mind at all. After all, she hadn't seen him for three months, and getting his long-awaited parcel seemed tho be the first time he had been happy since they had both arived home last Friday. Whatever it was on his mind would be a school thing, she thought, and Dick wasn't one to let on – he'd either tell her when he was ready or get over whatever it was.

Dick poured the acid through a glass funnel into the top bulb. The powder began to hiss and fizz. "Better shut the spare room, Dot," Dick said. "Mother won't want the smell going in there."

It was certainly pretty stinky! "Good thing we're outside," Dorothea said, laughing.

There was a "ding-dong" and a commotion inside as Mother went to the front door. "I better shut this down," said Dick. "I'll come and meet our surprise guest in a moment."

Dorothea rushed inside to where their apartment opened into the landing. "There you are Dot. This is my daughter, Dorothea. This is Mr Jenkyns, from Dick's school."