It might be a mainframe if:

• If you could kill someone by tipping it over on them, it might be a mainframe.
• If the only "mouse" it has is the one living inside it, it might be a mainframe.
• If you need earth-moving equipment to relocate it, it might be a mainframe.
• If you've ever lost an oscilloscope inside of it, it might be a mainframe.
• If it's big enough to be used as an apartment, it might be a mainframe.
• If it has ever had a card-punch designed for it, it might be a mainframe.
• If it weighs more than an RV, it might be a mainframe.
• If lights in the neighborhood dim when it's powered up, it might be a mainframe.
• If it arrived in its own moving van, it might be a mainframe.
• If its disk platters are big enough to cook pizzas on, it might be a mainframe.
• If Michael Jordan would need his entire annual salary to buy one, it might be a mainframe.
• If keeping all of the manuals together creates a fire hazard, it might be a mainframe.
• If it's so large that a dropped pen will slowly orbit it, it might be a mainframe.
• If it's ever been mistaken for a refrigerator, (or if the disk drive has ever been mistaken for a washing machine), it might be a mainframe.
• If anyone has ever frozen to death in the room where it's kept, it might be a mainframe.
• If it has a power supply that's bigger than your car, it might be a mainframe.
• If it has its own postal code, it might be a mainframe.
• If the operators considered the addition of COBOL to be an upgrade, it might be a mainframe.
• If it was designed before you were born, it might be a mainframe.
• If its main power cable is thicker than your neck, it might be a mainframe.
• If the designers have since died from old age, it might be a mainframe.