Ten-year-old Alzain Tareq from Bahrain became the youngest swimmer to compete at a World Championships when she took part in the 50m butterfly heats.
Tareq, whose mother is Scottish, finished last in 41.13 seconds, the slowest of all 64 swimmers in Kazan.
Her goal is to qualify for the 2020 Olympic Games in Tokyo.
“I was a bit nervous walking out there, I have never swum in front of so many spectators,” said Tareq, who got a photo with her idol Sarah Sjostrom.
“The other swimmers are often surprised, they ask me my name and how old I am and then they are like, ‘Are you swimming here?’
“I feel so happy. I want to learn the techniques and how they swim.”
The 10-year-old caused quite a stir in the media zone – and among the other competitors.
“I don’t know what to make of it – she’s dinky,” said Britain’s Fran Halsall, who was fifth fastest in qualifying for Friday evening’s semi-finals.
“Good on her for having a go.”
American star Missy Franklin said: “She is the cutest thing I’ve ever seen in my life. I got in her warm down lane the other day because I wanted to be friends with her, but I didn’t get a chance to talk to her.
“Every time she sees me she gives me the biggest smile.”
On day five of the eight-day meeting, Great Britain’s men’s 4x200m freestyle relay made it through to Friday’s final with the third fastest time.
Ben Proud reached the semi-finals of the 50m freestyle, while Lizzie Simmonds also progressed from the 200 backstroke heats.