LONDON—Mr. Hevisaurus, the lead singer of the Finnish heavy-metal band Hevisaurus, stepped up to the mic in front of 1,000 fans at 11:30 on a Wednesday morning.

“What’s a dinosaur’s favorite drink?” he screamed into the crowd. The crowd didn’t know. Mr. Hevisaurus tossed them clues: It’s white. It’s found in ice cream.

“Milk!” a voice yelled....