What have the guys in ONE DIRECTION been drinking on stage?!!

In a video from their concert in Louisville on Sunday, 21-year-old LOUIS TOMLINSON and 19-year-old NIALL HORAN APPEAR to drink something other than water from their water bottles...

Whatever they had in the bottle WAS clear . . . but when they took a sip, they reacted like it was MUCH MORE than water...

Niall responded to this yesterday by Tweeting, quote, "No way is that true!  Ever, Ever!  Who is making this [crap] up?"

Then three minutes later . . . after he'd apparently watched the video .
. . he posted another Tweet that said, quote, "Now I get why people are saying it.  Me and [Louis] do that every night!  [We] pretend there's  something in the water bottles."