... on land and sea, our answer is mighty and stern ...

Shop Think Geek pleased with another toy. A radio-controlled tank, which in a couple of seconds turns into an amphibian and, moreover, has a pneumatic gun, shooting at 20 meters.

The tank costs $ 60, is controlled on the ground and on the water, has dimensions of 235x158x215, a 700mAh battery and can flash red and blue LEDs when it turns from a land toy into a marine one.
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It seems I understood what I lacked as a child.