What is this for?

Tried replacing Zara's thumb with the pacifier today when she was half asleep and sucking on her thumb. She sucked on in for 30seconds, then found that something was not right, so she spit it out, and put her thumb back into her mouth. I tried one more time, she opened her eyes, took the pacifier out of her mouth with her hand and started examining the object. Her curiousity outweighed her sleepiness. She sat up, and had a closer look at the pacifier. She knocked it on the table to see what sound it could make, shook it to see if it rattled, then put the handle into the mouth to taste it. I tried to put the teat into her mouth, but she immediately pulled it out, and made further detail inspection of it. Daddy came by and started giving her some 'education' on what the object is for, but she just didn't seem to get it. When he put the teat in her mouth, she took it out, and smiled, or she just chewed on it for a few seconds and spit it out. We tried a few more times later, but she still wouldn't suck on it. I'm not so sure if we could ever replace the thumb with the pacifier. She must have forgotten in the earlier days, when she was a few weeks old, before she could stick her thumb into her mouth, this was something that we gave her to sooth her. Now to her, this is just a toy.