This animation is created using Masking Technique of Photoshop.
Open Photoshop 6.0. Create new file with 72 PPI and transparent background. Make the shape of the glass with Polygon lasso tool. Use grids for accuracy.
View > Show > Grid
Select desired foreground color.
Edit > Stroke (Select outside with 1 pixel).
Create new layer and drag it below the original.

Select upper layer and click in the top portion with Magic Wand Tool. Select Bottom layer and fill the selection with the desired color.

Keep the bottom layer image selected.
Layer > Add Layer Mask > Reveal Selection
The Layer palette will display the mask as follows.

File > Jump To > Image Ready 3.0
Animation palette of the file will contain only one frame.
Press 8th button from the bottom left which Duplicates current frame.
Observe the bottom layer carefully remove the link between the image and the mask by simply clicking on it.
Click the image icon (not the Mask icon) in the bottom layer. Select Move Tool and bring down the orange image at the bottom of the glass.

Drag the right side image to the left hence you will get empty glass in the left (1st Frame) and filled glass at the right (2nd Frame) in the animation palette.
Click on the black triangle at the upper right corner of the animation palette. Select Tween.
Select position and Tween the images with desired numbers of frame. The Images will be displayed in the animation palette as follows.

Set the timimgs for frames.
Set the no. of time you want to play the animation.
If you have any doubts about step 17 & 18, study Project 1 & 2 carefully.
After completing the animation try to check the performance in the browser
File > Preview In > Click the browser you want

Save the file with File > Save Optimized or File > Save Optimized As