Suppressing postback on a user control

I have a user control for a web site that shows a button called What Now. The page Load pulls the data that needs to be shown on this control, and the ascx page itself contains javascript to hide/unhide the control/tooltip. The problem, however, is that everytime you click the control to hide or unhide, it's making a server trip for some reason, and resetting. Anyone know why?

Here's the ascx page.
*************************
<div id="divWhatNowButton" class="what_now_top_bg" style="WIDTH:551px" onmouseover="ShowTip(23, event, 0)" onmouseout="HideTip(23, event)">
      <input type="image" id="imgWhatNowButton" height="33" width="551" src="images/top_center_title.gif" alt="Display/Hide the What Now help message." onclick="javascript:toggleWhatNow();">
</div>
<div align="left" id="divWhatNowText" style="WIDTH:552px;BORDER-BOTTOM:1px solid;HEIGHT:16px">
      <table border="0" cellpadding="5" width="100%">
            <tr align="left" valign="top">
                  <td><asp:label id="lblToolTip" runat="server" cssclass="instructions"></asp:label></td>
            </tr>
      </table>
</div>

<script language="javascript" type="text/javascript">
<!--
//toggleWhatNow();
function toggleWhatNow() {
//toggles display of "What Now" and sets the session cookie (not persistant)
      var iVisible      = readCookie('ShowWhatNow');
      var oToolTip      = document.all('divWhatNowText').style;
      alert('bVisible = ' + iVisible);
      if (iVisible == null) {
            iVisible = 1;
      }
      else if (iVisible == 0) {
            iVisible = 1;
      }
      else {
            iVisible = 0;
      }
      //alert('iVisible = ' + iVisible);
      if (iVisible == 1) {
            oToolTip.display = 'none';
      }
      else {
            oToolTip.display = 'block';
      }
      writeCookie('ShowWhatNow', iVisible);
      return false
}
//-->
</script>


And here's the vb code.
****************************
Public WhatNowID As Integer = 0

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Put user code to initialize the page here
        Dim bVisible As Boolean = False
        Dim sToolTip As String = ""

        If WhatNowID <> 0 Then
            sToolTip = clsMiscUtil.ToolTipGet(WhatNowID, ucHeader1.oSession)
        Else
            sToolTip = WhatNowID.ToString()
        End If

        lblToolTip.Text = sToolTip

    End Sub