Examples:

• When she saw her favorite celebrity in person, she felt weak at the knees with excitement.
• The breathtaking view from the mountaintop made him weak at the knees.
• His heartfelt declaration of love left her feeling weak at the knees.
• The unexpected news of her promotion left her weak at the knees with joy.
• The thought of public speaking always makes him weak at the knees.
• The intense heat of the desert sun made the hikers feel weak at the knees.
• Her dazzling smile had a way of making people weak at the knees.
• The majestic performance of the orchestra left the audience weak at the knees.
• Walking into the room filled with her childhood idols, she felt weak at the knees.
• The delicious aroma of freshly baked cookies made him weak at the knees.
• The sudden realization of the danger they were in left them weak at the knees.
• The roller coaster ride was so thrilling it left her weak at the knees.
• The touching tribute to her late husband at the memorial left her weak at the knees.
• The unexpected gesture of kindness from a stranger left him feeling weak at the knees.
• The romantic dinner by candlelight had her feeling weak at the knees.
• The heartbreaking news of her beloved pet's passing made her weak at the knees.
• The powerful performance of the lead actor in the play left the audience weak at the knees.
• The surprise proposal caught her off guard and left her weak at the knees.
• The sight of the newborn baby in her arms left her feeling weak at the knees.
• The sheer size and grandeur of the ancient cathedral made visitors weak at the knees.
• I have to admit that when we first met, and you recognized my perfume, you smelled it, I did feel a little weak at the knees.