The incident happened on a flight from Denver to Minneapolis on Sunday morning, activist Shannon Watts said. A United gate agent was "forcing" the girls, one of them aged 10, to change their clothes or wear dresses over the leggings, she tweeted. United said the girls were travelling on a ticket that had a dress code. They were "United pass travellers", which are tickets for company employees or eligible dependents, it explained in a Twitter exchange on the issue. Twitter storm Shannon Watts, founder of the group Moms Demand Action for gun reforms, tweeted about what happened to five girls when they tried to board a flight at Denver airport. She said three of the girls were allowed to fly after putting dresses over the top of their clothing, but two were prevented from boarding. She slammed the airliner for its actions, asking: "Since when does United police women's clothing?". Source: BBC