I looked and I said,hey, that's skateboarding on snow. That's perfect for me." And he's still at it 27 years later in a career that has included not only injuries like a broken collar bone, broken ankles and torn knee ligaments, but also a liver transplant.

VOA: standard.2010.02.26

So, imagine the pig has a heavy collar and to reward the pig for walking forward you might remove the heavy collar.

所以，想象这头猪带着很重的项圈,作为对这头猪向前走的奖励,你可以把项圈拿掉。

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