Are these spinach leaves?

Question

I often buy baby spinach from the store, and sometimes, in a few bags of spinach I also find these leaves:

They have a different shape than the spinach leaves, they are a lighter green, and they taste differently. The bag doesn't say that "it may contain traces of" other plants, so are these some part of the spinach plant? Or are they something different? Why do they often come together with the spinach leaves? I haven't found any other kind of leaves (besides spinach) in the bags except those in the picture.

Answer 1

Those are the cotyledons (seed leaves) of spinach.

For comparison, here's a picture of what spinach sprouts look like:

Source: https://wimastergardener.org/article/spinach-spinacia-oleracea/