levers是什么意思？

Well, first of all, he kept the two dolphins together in the same tank and taught them to press levers whenever they saw a light.

The levers were fitted to the side of the tank next to each other

If the light was kept steady, the dolphins were supposed to press the levers in reverse order.

The levers and light were set up in exactly the same way except that this time it was only Doris who could see the light indicating which lever to press first

But in order to get their fish, both dolphins had to press the levers in the correct order.