PHILADELPHIA—A reciprocated expression of gratitude provided upon receiving a thank-you note last month has plunged friends Amy Hildenbrand, 34, and Melissa Means, 35 into a recursive appreciation spiral that has no apparent end in sight. "Melissa was so happy my note was hand-written that she insisted on buying me dinner," said a visibly frustrated Hildenbrand, who countered by offering to babysit Means' infant son and was rewarded with a heartfelt letter and gift card. "If you'll excuse me, I need to get to the travel agent before it closes." Although the all-encompassing vortex of thankfulness has left both parties unable to move for fear of being further indebted, Hildenbrand will finally put the matter to rest next Thursday, when she attends her surprise Thank You luncheon with a loaded shotgun.