What makes Roy think Beatrice is an expert at lying to adults?

Beatrice winds his mother around her little finger.... she has the perfect explanation for everything.

"It's a science experiment," Beatrice cut in. "It might get pretty messy."
Roy suppressed a smile. Beatrice had sized up his mother perfectly: Mrs. Eberhardt kept an exceptionally neat house. Her brow furrowed at the thought of glass beakers bubbling with potent chemicals.
"Is it safe?" she asked.
"Oh, we always wear rubber gloves," Beatrice said reassuringly, "and eye goggles, too."
It was obvious to Roy that Beatrice was experienced at fibbing to grownups. Mrs. Eberhardt fell for the whole yam.