The week after Thanksgiving I went in for a prenatal visit, the first of which where I heard the baby's heartbeat. I took Colin with me, and sat him in the chair next to me for the 30-ish seconds that I even saw the doctor that day.
I didn't think much of it, but apparently Colin was paying close attention. Later that day while we were playing in his room he found his toy stethoscope, put it in his ears, then made me pull my shirt up over my stomach so he could listen to the baby's heartbeat like we did at the doctor's office :)