Not enough strength
At first, there doesn’t seem to be a problem. Mother bear and her cubs jump into the water and swim a good long way towards the other side of the lake. Soon, though, it becomes clear that the two cubs can’t keep up with their mother. They try their best and use all of their strength, but the distance between the mother and the cubs grows bigger and bigger. When the mother notices she has lost her cubs, she turns back so the cubs can cling to her to not drown in the lake. The cubs aren’t strong enough yet and they can’t finish the journey. The crossing is going to end very sadly.
Letting go
It should really be the other way around and, usually, that is the case in nature. A mother should sacrifice herself for her cubs. But, this mother doesn’t have a choice, unfortunately. She can’t keep the cubs on her back and they can’t finish the journey on their own either. The mother bear only has one option: keep swimming and leaving her cubs behind. It’s shocking and unimaginable, but sometimes there’s no other choice. In nature, everything is about surviving, after all.