Are acorns toxic in any way to dogs?

This morning, Mira comes charging in from outside and drops something on the floor that she's obviously VERY pleased with. From a distance, I thought she'd brought a small mouse or a big bug into the house. Turned out to be a moderately sized acorn. She was as excited about it as she would be over a new toy or live mouse. (No, I don't give her live mice, but she's found them on her own in the past.) I have no idea why an acorn, of all things, evoked such a reaction. Maybe it was soaked in squirrel urine? LOL I took it from her and gave her a suitable treat in exchange.

Now I'm concerned. If she plays with acorns, she'll eventually try to eat one. And if she likes it, she'll eat more. We have over 30 oak trees in the backyard, plus countless more hanging over from adjacent properties. No way to keep them "picked up" or anything.