Karen O of the Yeah Yeah Yeahs treats her role as rock star like a child giddy and hypnotized at a toy box.

She extends the microphone away from her body and  sweepingly presents it to the audience like a magical object she found buried in the garden. Microphone, oh, oh! Smile extending across her lips.

She arrived Saturday night donning a flowing palm-print headdress, with fluorescent colors wildly expelling from high above her head to the floor. She removed it with dramatic relish and showed off her pretty tights.

Karen O dances with no regard for what dancing or choreography looks like.

"We weren't even supposed to be here!" she explained to a crowd that knew all too well that the group was the last minute addition to the Lollapalooza schedule, after the Beastie Boys canceled due to Adam "MCA" Yauch's health issues.

The show wasn't without its missteps. One was forgivable - some tech misfire made for an awkward restart of "Hysteric." The other? Miss O forgot her own lyrics to the band's biggest hit, "Maps," even when played at tonight's super-slow tempo. The crowd made up for it and Karen, of course, just giggled a lot.

Meanwhile, our spies across the park reported that Tool put on a show fairly comparable to its visual-heavy set at All Points West, with dark projections and videos by guitarist Adam Jones. Also, front-freak Maynard Keenan stripped to his tighty whities - or rather, black boxers - and declared the mucky hot day to be a clothing-optional night.

The proggy hard rock band finished off with no encore but apparently melted off what was left to be melted of its fans' minds with "Vicarious."

Ben Harper had a good day with his still-new backing band Relentless 7 (three of them, really) and stuck with newer, hard-rocking material from his latest "White Lies For Dark Times." Harper was mainly stone-faced, while watching his drummer was like witnessing a wild cat devour prey - impossible to look away, savage.

TV On The Radio featured Tunde Adibempe in full-gospel mode, complete with a white button-down, with the volume increased by an impressive horn section. Kyp Malone's facial hair and Afro continues to shows no bounds.

Santigold stole the afternoon, with stoic backup /singers dancers, wearing and one of the ugliest and grooviest jumpsuits in history. She's a giving and electrifying performer, stepping to the edge of the stage and beaming during hot tracks like "Starstruck." We think we saw GZA in the wings.

The fans were most lively further in the middle of the Grant Park, bouncing to Animal Collective after the sun mercifully descended. They were noisy and put on a nice light show.

Lykke Li yet again covered Kings Of Leon's "Knocked Up." But she didn't leave it at that. Ever wonder what it's like to hear a petite Swedish woman perform Lil Wane's "A Milli?" Check YouTube in the morning.