Papa Bear took his two sons to the other side of the river to climb mountains. There was only one empty boat on the river, and the boat could carry a maximum load of 100 kilograms. Papa Bear weighed exactly 100 kilograms, and the two sons each weighed 50 kilograms. How can they all cross the river?
熊爸爸带着两个儿子去河对岸爬山，河上只有一只空船，船最多能载重100千克，而熊爸爸正好重100千克，两个儿子各重50千克．问他们怎样才能全部过河？
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