This is the stuff that Millennial Music Festival Dreams Are Made Of!
The singers — who are hitting the road together in September on Halsey's Hopeless Fountain Kingdom World Tour — joined forces for a surprise Spice Girls cover during Charli's Lollapalooza set.
Ever the sneaky one, Halsey dropped a pretty sizable hint about the duet via Instagram shortly before she joined Charli and the zig-a-zig-ahs kicked in.
Charli first teased the team cover on her Instagram, posting a pic of her and Halsey together with the caption, "....... are u ready lolla?? #zigazigahhhhh":
Watch their impromptu performance in full below and prepare to be singing your heart out!