Web Comic Update

So a physicist, a chemist, and biologist are living together on a farm with a cow. One day, the cow stops giving milk. The biologist first looks and the cow, and after some consideration declares, "It must be the cells!". Next, the chemist takes a look at the cow, and after thinking for a while exclaims, "No, the problem is that the cow has an imbalance of biochemicals!" Finally, the physicist takes a look at the cow, and after careful contemplation says, "I think I've found the reason why the cow has stopped giving milk, but it only applies to spherical cows in a vacuum!" *cue drum*

Know any decent (or bad) physics jokes appropriate at the bachelor level?