Holding the sign they made for their lemonade stand, Julie Jacobs and Chase Anderson, both 13, described how their summer of fun turned ugly fast.
"There were people stopping. There were people turning around because they wanted lemonade and it just happened," Chase said.
They said a man passed by their stand on a corner at Mountain View Drive and Terra Vista Parkway.
Top news of the day
"He said he dropped his dollar on his run so we gave him a free glass and then he started walking away and then he came back around ... and grabbed the box," Julie said.
The box had $20 in sales they had made and Chase's wallet with another $10.
Her brother and a friend, who were there helping the girls, saw the teen run and chased after him.
Soon after, a deputy arrived and detained the suspect who police say had barely turned 18.
"When we search him he's got the $30 on him," said Teresa McMahon, a Rancho Cucamonga Police Department spokeswoman.
Police believe the teen took a cellphone from a young man before taking the lemonade money.
"He walked up to the juvenile, said 'Hey, let me borrow your cellphone,'" McMahon said. "The kid unlocked the cellphone and the suspect took off with it."