<div class="box">
        <input type="text" id="showValue" class="text" />
        <table class="table">
            <tr>
                <td><input type="button" class="d0btn" value="退格" /></td>
                <td><input type="button" class="d0btn" value="清屏"/></td>
                <td><input type="button" class="d1btn" value="sqrt"/></td>
                <td><input type="button" class="d2btn" value="÷"/></td>
            </tr>
            <tr>
                <td><input type="button" class="d3btn" value="7" /></td>
                <td><input type="button" class="d3btn" value="8" /></td>
                <td><input type="button" class="d3btn" value="9"/></td>
                <td><input type="button" class="d2btn" value="×"/></td>
            </tr>
            <tr>
                <td><input type="button" class="d3btn" value="4"/></td>
                <td><input type="button" class="d3btn" value="5"/></td>
                <td><input type="button" class="d3btn" value="6"/></td>
                <td><input type="button" class="d2btn" value="-"/></td>
            </tr>
            <tr>
                <td><input type="button" class="d3btn" value="1"/></td>
                <td><input type="button" class="d3btn" value="2"/></td>
                <td><input type="button" class="d3btn" value="3"/></td>
                <td><input type="button" class="d2btn" value="+"/></td>
            </tr>
            <tr>   
                <td><input type="button" class="d3btn" value="0"/></td>
                <td><input type="button" class="d3btn" value="."/></td>
                <td><input type="button" class="d3btn" value="+/-"/></td>
                <td><input type="button" class="d4btn" value="="/></td>
            </tr>
        </table>
    </div>

        var firstValue = "";
        $(function () {
            $("input:not(#showValue)").mousedown(function () {
                $(this).attr({
                    style: "background-position:0px 0px"
                }).mouseup(function () {
                    $(this).attr({ style: "background-position:0px -44px" });
                }).click(function () {
                    if (this.className == "d3btn") {
                        firstValue = this.value.toString();
                        alert(firstValue);
                    }
                });
            });
        });