To predict the sinking/floating behavior of a spherical object, you could determine its density.
<em>Step 1</em>. Determine its <em>mass</em> on a suitable balance.
<em>Step 2</em>. Determine its <em>volume</em>
<em>Either
</em>
Measure its circumference. <em>C</em> = 2π<em>r</em>, so you can calculate r
<em>or
</em>
Measure its diameter. <em>d</em> = 2<em>r</em>, so you can calculate <em>r
</em>
<em>Then
</em>
<em>V</em> = ⁴/₃π<em>r</em>³
, so you can calculate its volume.
<em>Step 3</em>. Calculate its <em>density </em>(<em>D</em>).
Density = mass/volume
If D < 1 g/cm³, the object will float in water. If D > 1 g/cm³, the object will sink.