Start by opening an image of a glass.

Create an ellipse using elliptical shape tool. Reduce the opacity to 20%. Ctrl+Click the layer thumbnail to get the selection. From select menu apply modify> >contract. Contract the selection by 1 pixels. Create a new layer and apply a black to white gradient in linear mode. Change the layer mode to screen. Merge both the layers. Press Ctrl+T. Right click the layer and select warp. Adjust nodes to get the shape as shown. Double click the layer to open layer style window. Apply layer style with the settings shown. Change the layer mode to screen and reduce the opacity to 51%. Create several duplicates of the drop. Press Ctrl+T and distort. Place as shown in the image below. Open animation window. For the first frame hide all the drop layers except the top drop layer. Duplicate frame and hide the first drop layer. Unhide the next drop layer. Similarly hide unhide the layers to create a falling drop effect. Press Ctrl+Alt+Shift+S to save an optimized animated GIF.