I’ve seen another version of this with a funny (but unnecessary 2nd punch line). This version is funny on its own. If this particular incident happened to you and I have broken confidentiality (j/k), I apologize.
One year at Thanksgiving, my mom went to my sister's house for the traditional feast. Knowing how gullible my sister is, my mom decided to play a trick. She told my sister that she needed something from the store. When my sister left, my mom took the turkey out of the oven. She removed the stuffing, stuffed a Cornish hen, inserted it into the turkey, and re-stuffed the turkey. She then placed the bird(s) back in the oven.
When it was time for dinner, my sister pulled the turkey out of the oven and proceeded to remove the stuffing. When her serving spoon hit something, she reached in and pulled out the little bird.
With a look of total shock on her face, my mother exclaimed, "Patricia, You've cooked a pregnant bird!"
At the reality of this horrifying news, my sister started to cry. It took the family two hours to convince her that turkeys lay eggs!