What trick did Chuck play on Gally in Ch. 5

Chuck placed a wooden crate outside of the bathroom and tapped on the window. Gally's response was to open the window and ask who was knocking, which was Chuck's cue to jump and and scream, causing Gally to fall.

"The bathroom," Chuck whispered.

"So?" A thread of unease stitched along Thomas's skin.

"I love doing this to people. Gives me great pleasure before bedtime."

"Doing what?" Something told Thomas Chuck was up to no good. "Maybe I should — "

"Just shut your mouth and watch." Chuck quietly stepped up onto a big wooden box that
sat right under the window. He crouched so that his head was positioned just below where
the person on the inside would be able to see him. Then he reached up with his hand and
lightly tapped on the glass.

"This is stupid," Thomas whispered. There couldn't possibly be a worse time to play a
joke — Newt or Alby could be in there. "I don't wanna get in trouble — I just got here!"

Chuck suppressed a laugh by putting his hand over his mouth. Ignoring Thomas, he
reached up and tapped the window again.

A shadow crossed the light; then the window slid open. Thomas jumped to hide, pressing
himself against the back of the building as hard as he could. He just couldn't believe he'd
been suckered into playing a practical joke on somebody. The angle of vision from the
window protected him for the moment, but he knew he and Chuck would be seen if
whoever was in there pushed his head outside to get a better look.

"Who's that!" yelled the boy from the bathroom, his voice scratchy and laced with anger.
Thomas had to hold in a gasp when he realized it was Gaily — he knew that voice already.

Without warning, Chuck suddenly popped his head up toward the window and screamed
at the top of his lungs. A loud crash from inside revealed that the trick had worked — and
the litany of swearwords following it let them know Gaily was none too happy about it.
Thomas was struck with an odd mix of horror and embarrassment.