>But ours don't fit INTO lorries--I mean trucks!

Nearly right Jackie - not into (there wouldn't be room for many!) but onto - there is a special clippy thing that clips them on, so they can be tipped up into the lorry as described by others above. Some areas have different coloured ones for recycling but Edinburgh, for some reason known only to the good burghers is in the dark ages in that respect.

PS Funny that the Cambridge Dictionary chose Edinburgh for it's example http://dictionary.cambridge.org/define.asp?key=burgher*1+0 - are they trying to tell us something?