documentclass[12pt]{article}
 usepackage{tikz}
 usetikzlibrary{calc,shapes.callouts,shapes.arrows}
 newcommand{bubblethis}[2]{
    tikz[remember picture,baseline]{node[anchor=base,inner sep=0,outer sep=0]%
    (#1) {underline{#1}};node[overlay,cloud callout,callout relative pointer={(0.2cm,-0.7cm)},%
    aspect=2.5,fill=yellow!90] at ($(#1.north)+(-0.5cm,1.6cm)$) {#2};}%
}%

 begin{document}
 bubblethis{B}{I know X}
 end{document}

documentclass[12pt]{article}
 usepackage{tikz}
 usetikzlibrary{calc,shapes.callouts,shapes.arrows}
 newcommand{bubblethis}[2]{
tikz[remember picture,baseline]{node[anchor=base,inner sep=0,outer sep=0]%
(#1) {#1};node[overlay,cloud callout,callout relative pointer={(-0.2cm,-0.2cm)},%
aspect=1.5,fill=yellow!90] at ($(#1.north)+(0.8cm,0.4cm)$) {#2};}%
}%

newcommand{bubble}[3]{
tikz[remember picture,baseline]{node[anchor=base,inner sep=0,outer sep=0] %
(#1)  {#1};node[overlay,cloud callout,callout relative pointer= {(0.2cm,-0.4cm)},aspect=2.5,fill=red] at($(#1.north)+(-0.5cm,1.6cm)$) {#2};node[overlay,cloud callout, callout relative pointer={((-0.4,-0.3))},aspect=3,fill=green] at($(#1.north)+(2cm,4cm)$) {#3};}


}

begin{document}
vspace*{4cm}
centering
bubble{B}{I know}{bubblethis{A}{B}}
end{document}