The day is hot, which means Patricia Mertz, who sells cold lemonade, cherry fizz and root beer, is in business. The counter of Little John is filled with barrels and corked or wire-capped bottles of the homemade drinks, made right here behind the tent.

She owns Little John’s which brews elixirs on site, behind the tent. Customers buy a small, medium, large or extra-large bottle and get it filled from a barrel. When they want more they bring the bottle back .

Mel Thiesen, a retired nuclear plant computer operator who works for Mertz, pours a bucket of dry ice into a barrel, where it gurgles and smokes. The dry ice keeps the drinks cold and carbonated,.

Mertz, who lives in Kansas City, has owned Little John’s for 24 years and does reenactments across the Midwest, South and mid-Atlantic.

To keep it authentic, she sells glass bottles rather than plastic, and customers pay $1-$4 for the refills. Her most popular item is root beer, which was just as popular during the Civil War.

“When we first started we were the only soft drink you could get,” Mertz says. “Coca-Cola wasn’t allowed.” But at Camp Manassas, a vendor sells Coca-Cola about 50 feet away.

But today is supposed to be the hottest day of the year, which may very well drive away spectators. Thiesen said 83 to 93 is the optimum temperature, cool enough to get people out but hot enough to keep them thirsty.

One parched customer has already returned twice in 15 minutes.

The Post’s Timothy R. Smith will be blogging all day from the events in Manassas honoring the 150th anniversary of the Battle of Bull Run.