OK, being an engineer, I couldn't resist analyzing this. First, we need to make some reasonable assumptions, based on some average standards out there. I will use SI measurements, but you can easily convert to the unit of your choice. First, there are several studies that report the average volume of ejaculate is between 0.1-10ml. This is the total volume. An average is about 5 ml total. Let's say that is equally divided over 5 shots--this varies a great deal, I know, but for the sake of calculation. This gives one shot as 1 ml. Let's make a best case assumption for this exercise, that the semen has the same density as water---it will never be lower than water, and is usually higher. This gives the mass of one shot as 1 gram. Now, checking around a bit, an average height of a non pillow top mattress plus box spring is about 2 feet high, or about 0.61 meters. Here in the US, the average ceiling height is 8 feet, or about 2.44 meters. This means that the sperm must travel vertically a height of 1.83 meters. Let's subtract off the thickness of the person and his dick--let's say, 40 cm. This gives the height it must travel upon leaving the end of the penis of 1.43 meters to JUST touch the ceiling. Now, the work required to raise 1 gram 1.43 meters against the force of gravity, neglecting air friction, etc. is W=mgh, giving the energy in joules, mass in kg, and heigh in meters, g=9.81 m/s^2. Thus, W=(.001kg)(1.43m)(9.81m/2^2)=0.014 Joules. To do this, kinetic energy must be converted to potential energy, so kinetic energy= 1/2mv^2. Running the calcs, this means that the shot has to be traveling about 7.5 meters/second. This equates to about 2.1 km/hour. Now, the shot starts from zero velocity, and has to accelerate to the 7.5 m/s. So, let's assume a contraction lasts about 0.5 seconds. This gives an acceleration of 15m/s^2. Now, Force=(mass)(acceleration). So, the force is F=(0.001kg)(15m/s^2)=0.01 Newtons. This is about 0.0023 pounds of force. Now, to put that much force against the gram through the urethra requires a reasonable pressure. Let's assume the urethra is about 3 millimeters in diameter--fairly reasonable. Now, let's assume a circular cross sectional area, so the area the force is applied over is PIr^2, or putting in the numbers, about 7.1 square mm, or 7.1x10-6 square meters. So, substituting in again, we get a pressure of 0.01 N/7.1x10-6 sq. m=1408.5 N/m^2. This equates to about 0.2 pounds per square inch. That's really not an insanely high pressure, or an insanely high velocity, or acceleration. So, hitting the ceiling from lying on the bed is not out of the realm of possibility. According to Kinsey, the average speed of ejaculation s about 28 MPH. That's about 12.5 m/s. This is far greater than our calculated 7.5 m/s velocity needed to just hit the ceiling. So, the question really is, why aren't MORE of you guys hitting the ceiling? :biggrin1: Please feel free to correct any of my math or assumptions here--it has been a long time since I have done any kinematics...:smile: