Steam
The water from a tsunami can create a lot of energy when made into steam. Steam power used to be all the rage back in the day. Steam comes from water, water comes from tsunamis, so logically, tsunamis are a great source of energy.
Imagine how much steam a tsunami could create. Hear the steam hissing off of the water and feel the moisture from the steam washing over your body.
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In order to contain this steam energy and store it for later, you lock it up in a box with a key (き). That way the energy can be used at a later time. Provided you don’t lose the key, that is.
Steam is very light, so carrying this box of steam energy will be no problem. Picture yourself wandering about with a box of steam in one hand and a key in the other.