Haven't tried any too difficult ones, we've had a tug-a-jug that was a flop, they really couldn't figure it out. A kong wobbler-Ripley smashed this thing around like crazy making a ton of noise, she loved it, I didn't like finding kibble under everything forever. The other two not so much, they aren't as forceful so food didn't come out. Kongs the girls like and I freeze for them sometimes, but Nero will just leave it untouched.