Technical question about nucleation of beer!? I work as a barman and am also a physics student. But I just can't seem to get my head around something: I understand that beer (referring to lager really) consists of water with CO2 dissolved in it with other, irrelevant, stuff also. You cannot see the gas because it is dissolved; however if the glass it is in contains a nucleation point (a widget) the gas is released as bubbles which float to the top of the beer and stay there. However, sometimes bubbles do not form i.e. you pour the pint and there are no bubbles being formed afterwards. My managers tell me this is because either the glass is warm, wet or dirty. But they cannot tell me WHY. Surely, if the glass is warm, the gas should be released more readily; I cant understand why it being wet would have much effect; If it is dirty, nucleation should still occur. Also, if you tap the glass the glass a flat pint is in such that it vibrates, bubbles form for a very short moment (presumably whilst the glass is stil vibrating) some bubbles form. Can somebody please tell me why pouring lager into a widget glass that is warm, dirty or wet somehow 'deactivates' the widget unless you tap it. Thanks!