It was tiny, probably round and definitely hard.
And it came through the toilet window, leaving a small hole with cracks around it.
It could have hit junior college student Karan Singh Randhawa.
Fortunately, the 17-year-old, who was washing his face at the sink, had lowered his head just as the object came flying in, likely at great speed. (See report on right.)
The sound, a loud "tak", surprised him. He turned around, saw a hole in the window and quickly walked out of the toilet.
Karan recounted: "I was a bit scared. I didn't know what was going on and the sound was quite loud. After that, I could hear the sound of glass dropping onto the floor."
The incident happened at The Tropica condominium in Tampines around 6.50pm on Saturday.
Karan and his family live in a first-storey unit.
The teen said the "tak" sound was so loud that his grandmother, who has a minor hearing problem, could hear it from the living room. Madam Piaro, 87, recounted: "It sounded like somebody shooting. It (the force) must have been very strong to break the glass."
Glass shards were found near the living room - 5m from the toilet window - and they missed Madam Piaro by 2m. Karan then went outside his apartment to check.
He said: "The only people I saw were two or three six-year-olds playing football a distance away. The hole (about half a centimetre in diameter) couldn't have been made by the ball because the hole is really small."
Karan informed his mother and she asked him to call their neighbour for help as she was busy.
Their neighbour, who gave his name only as Mr Paul, 55, was alarmed when he saw the damage.