The strange coffee

Mary watched the last drop of coffee slide out of her cup and on to the floor.
There was a sizzle, then a whiff of steam. The drop of coffee burned through the tile to the basement.
Mary looked at the coffee pot, swirling it carefully. She’d used a free sample she’d received in the mail.
After a moment of panic, she realized it hadn’t burned a hole through her.
She went into the bathroom and checked to make sure.
After she got dressed again, she shrugged and filled another cup.
This time, she added sugar, igniting a massive explosion.