A snapshot at time t=0:

Clear[f,x,y,t,time];
f[x_,y_,t_]=20+Sin[x+y+t];
time=0;Plot3D[f[x,y,time],{x,-7,7},{y,-7,7},PlotPoints->24,AspectRatio->1,ViewPoint->{1.3`,-2,0.5`},PlotRange->{0,22},AxesLabel->{"x","y","f"}]

?Animate

A ``movie'':

Clear[f,x,y,t,i];
f[x_,y_,t_] = 20+Sin[x+y+t];
Animate[fred[i]=Plot3D[f[x,y,i/4],{x,-7,7},{y,-7,7},PlotPoints->24,AspectRatio->1,PlotLabel->{"=t"N[i/4]},
PlotRange->{0,22},ViewPoint->{1.3,-2,0.5},AxesLabel->{"x","y","f"}]
,{i,0,24,1}]

*******************************************

These are the graphs of the level surfaces for the temperature function Tami showed in class.

Clear[f,F,x,y,z,c];f[x_,y_,z_]=69.5`+200 (E^(-(((x^2)+(y^2)+4 (((100-z))^2))/1000)));c=70;F[x_,y_,z_]=f[x,y,z];

c=70;
Print[" "];
Print["temp = ",c];temp70=ContourPlot3D[F[x,y,z]==c,{x,-100,100},{y,-100,100},{z,0,100},Axes->True,PlotRange->{{-100,100},{-100,100},{0,100}}]

c=80;
F[x_,y_,z_]=f[x,y,z];
Print[" "];Print["temp = ",c];temp80=ContourPlot3D[F[x,y,z]==c,{x,-100,100},{y,-100,100},{z,0,100},Axes->True,PlotRange->{{-100,100},{-100,100},{0,100}}]

c=90;
F[x_,y_,z_]=f[x,y,z];
Print[" "];Print["temp = ",c];temp90=ContourPlot3D[F[x,y,z]==c,{x,-100,100},{y,-100,100},{z,0,100},Axes->True,PlotRange->{{-100,100},{-100,100},{0,100}}]

c=100;
F[x_,y_,z_]=f[x,y,z];
Print[" "];Print["temp = ",c];
temp100=ContourPlot3D[F[x,y,z]==c,{x,-100,100},{y,-100,100},{z,0,100},Axes->True,PlotRange->{{-100,100},{-100,100},{0,100}}]

Show[temp70,temp80,temp90,temp100]

*******************************************