He and his four classmates have barely learned to speak even in their native Spanish, but already they are absorbing Mandarin Chinese -- as are many adult Spaniards concerned for their job prospects.
"Xiang jiao," Leon replies in a tiny voice, grinning as he is rewarded with a bite of banana and a sticker. "Xie xie. Thank you."
Fu's class -- offered free for the first month -- is the newest after-hours activity for children at the TEO private nursery, whose parents hope it will pay off later in life.
Numerous schools and language centres here have started holding such lessons as Spaniards look to China's fast-growing economy for opportunities after five years of on-off recession in Spain.
"Chinese seems to me an essential language in today's world, and the best way to learn it is from an early age. Learning it as an adult seems much more difficult," says Leon's mother, Sara Vergara.
'A long-term strategy'
"It is a long-term strategy, for his job prospects in the future," adds Vergara, a 33-year-old housewife, arriving to pick Leon up from the class. "And I think he is enjoying himself."
Pilar Alvarez, director of TEO, said the nursery launched the after-hours Chinese lessons after seeing that many other schools in Madrid were doing so.
"After the second or third class, the kids start really getting into it," she says. "We are considering introducing it bit by bit for all the children during normal school time."
Regional governments in Spain are also expanding Chinese courses in their subsidised language centres, while some public schools are offering them as an after-school activity.
A programme of free classes jointly funded by the Andalucia government and the Chinese state has seen enrolments nearly double since it started two years ago, to 1,200 for this school year, the regional education ministry says.
It estimates that 30,000 people are currently studying Chinese as a foreign language in Spain. No such figure was available from the national education ministry.
"China is expected to be the leading world power in a few decades," the Andalucia ministry said in a statement. "This is driving a boom in the number of people studying its language and culture."
Madrid's network of official language schools has taught Chinese since the 1960s but demand has surged recently, said Maria Jose Garcia-Patron, head of secondary education and professional training in the regional education ministry.
"Demand for these lessons was stable for 40 years, with about 80 or 90 students enrolled, but over the past 10 years the number has grown markedly and has reached about 300," she told AFP in an email.
The recent crop of students in Chinese seem undeterred by its alien systems of intonation and writing that many see as challenging for Western learners.
"It is a bit hard to write, but I think it is easy to teach children to talk," said Fu. "Children have good memories."
Fu, 25, came to Spain six months ago and applied for the teaching job with Bambu Idiomas, a private company that organises classes for schools and individuals of all ages.
"There are lots of opportunities in Spain. Lots of families are looking for Chinese teachers, and now lots of nurseries too," she said.
Set up in 2011, the family-run company had 87 pupils signed up last year. This year the number surged to 235, said one of its Spanish founders, Ruben Camarero.
"It is an important language for the future," he said. "We decided it was a language that would interest people because Spain is in an enormous economic crisis and China is drawing a lot of interest worldwide."
In the classroom, Fu plays from her laptop the nursery rhyme known in Europe as "Frere Jacques", sung in Mandarin in a version well-known to Chinese children.
As she repeats the names of fruit to the five toddlers, correcting their intonation, four-year-old Angela jumps around excitedly, her long brown hair whirling.
"Banana!" she yells. "Xiang jiao!"