| 摘要 |
A board includes a number of cavities enclosed by a plate, to slidably receive a number of magnetic members, each of the magnetic members includes an outer end facing toward and movable toward the plate. A magnetic pen is movable on the board, to attract selected magnetic members toward the plate, and to have the outer ends of the selected magnetic members to be selectively seen through the plate. A liquid may be received in the cavities of the board to shield the magnetic members, and to prevent the magnetic members from being seen through the plate when the magnetic members are retracted into the cavities of the board.
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